You need Disk Management when you add an unformatted hard disk drive (removable or local) to your computer, want to change drive letter for a drive or partition, or change partition size.
The terms "partition" and "volume" mean the same thing in this article and are used interchangeably.
Starting Disk Management in Windows
Right-click (or tap and hold) Computer or This PC icon on Desktop or Start menu/screen and choose Manage.
In Windows 8, 8.1 and 10, you can use keyboard shortcut Windows Key+X to open Quick Links menu (a list of system tools) and click Disk Management there.
Windows Vista users should click Continue in the most beloved User Account Control window.
Computer Management window opens. Click Disk Management in the left pane, Storage section.
Initializing and formatting new disks in Windows
The most usual case is adding a new, unformatted hard disk. It may be your new external USB hard disk drive (although majority of these came preformatted now) or a new secondary local hard disk for large files, such as videos. Windows does not assign a letter to an unformatted disk, you will have to initialize it first, add a partition or volume and then format it.
Windows creates partitions as volumes, so you also have the ability to reduce and increase the size of partitions on the fly, without losing your data. Well, in Windows Vista and later, at least. 
Second case is connecting an NTFS-formatted drive from another device. This only requires initialization, formatting is not necessary and must be skipped in order to preserve the data on the drive.
If you have added a new (unformatted or formatted) hard disk drive to your computer, the initialization wizard starts right after opening Disk Management.
In Windows XP, click Next in the Initialize and Convert Disk Wizard.
In Windows Vista and newer, make sure your new disk is selected under Select disks list in the Initialize Disk wizard. In most cases, leave MBR (Master Boot Record) partition style selected.
For large disks (more than 2 TB (terabytes) in size), select GPT (GUID Partition Table) instead. Please note that older Windows versions, such as Windows 95, 98, Me, NT and 2000 cannot access GPT partition style or partitions larger than 2 TB. Only Windows XP and later support GPT.
In Windows XP, just tick the check box for your new drive. There are no additional options available.
Click OK (Windows Vista and later) or Next.
In Windows Vista, 7, 8, 8.1 and 10, a new basic disk will be created and the initialization process is complete. If the new drive was preformatted, it gets a drive letter automatically and you do not need to take any further actions.
Windows XP Professional (not Home!) offers to convert the new disk to a dynamic disk. This is not necessary, please clear all check boxes before clicking Next.
Disk initialization in Windows XP is now complete. Click Finish.
If the new drive was preformatted, it gets a drive letter automatically and you do not need to take any further actions.
Creating and formatting partition on a disk
Formatting is required for new, unformatted disks only. If you just initialized a drive that was used with another computer/device and the drive contains data, skip this step altogether - the drive is ready for use.
An unformatted disk needs to have at least one partition to become usable. In Windows Vista, 7, 8, 8.1 and 10, partitions are created within volumes and you start by creating one.
To create a new partition, find your new disk from the bottom part of the Disk Management console. It usually contains word "Unallocated" in it.
Right-click or tap and hold on the disk and select New Simple Volume (in Windows Vista and later) or New Partition (in Windows XP).
New Simple Volume Wizard or New Partition Wizard (XP only) opens. Click Next.
Only Windows XP offers a choice between Primary and Extended partition. Leave Primary partition selected and click Next. Home users do not normally need Extended partitions.
In most cases, it is best to allocate all available disk space to the new volume. Click Next again.
Next you need to assign a drive letter to the new partition. By default, Windows Vista and newer offer the first available drive letter not in use by any other local or network drive. Windows XP omits the counting of network drives, so you might see a warning that the drive letter is already mapped to a network share. In such case, simply choose another drive letter.
Drive letters "A" and "B" are reserved for floppy drives in Windows XP and older, "C" is usually the drive where your Windows installation, programs and documents are, and "D" is probably your CD/DVD drive or some recovery partition in Windows 7 and later.
As the selected drive letter is probably fine, click Next here.
Click Format this partition with the following settings and specify parameters.
- File system - NTFS is the best choice for the hard disks that are inside your computer case (local disks) and for external disks that you want to use for backups. If you are formatting some external hard disk or USB drive and you also want to connect it to smartphones, photo cameras or really old Windows 9x machines, select FAT32 instead - this improves compatibility.
For USB flash drives, FAT32 is mostly the best selection, unless you need to transfer files larger than 4 gigabytes. For the latter case, select NTFS. - Allocation unit size - in most cases, leave Default selected. Default is 4096 bytes (4 kilobytes). This works best for disks that contain all sorts of files - documents, pictures, videos, etc. If you are planning to use the drive only for large video files or backups, you should select 16K (kilobytes). This improves file performance for large files, but this also wastes loads of disk space for small files, so put only large files on that disk!
- Volume label - this is a descriptive name for your new drive/partition - Videos, Backups, etc. You can also leave this field empty, but this is not recommended.
The label is visible in My Computer/This PC and in tools such as DiskPart. All unlabeled drives are listed as "Local Disk" or "Removable Disk" (depending on their type) and this might make distinguishing between drives more difficult. You can also set a name for any existing partition or drive without losing data later. - Perform a quick format - always check this box, this decreases formatting time drastically.
- Enable file and folder compression - if you are formatting a local (not removable) disk with NTFS file system and the disk will contain mainly usual documents (spreadsheets, pictures, presentations, etc), you can turn this on. This will compress the files and you can use more space than your drive offers - 2 to 10 times for Microsoft Word documents, Excel spreadsheets, PowerPoint presentations and plain text files, much less for programs, photos (jpg, png, gif) and music files (mp3, aac). Compression will not mean a noticeable reduction in disk speed.
If you will use the disk for video files, compression has no effect and you can leave the box unchecked.
You cannot use file and folder compression if file system type is not NTFS or if allocation unit size exceeds 8192 bytes (8 kilobytes).
In most cases, the defaults are fine, so click Next.
The wizard has now asked all its questions, click Finish.
After this, your new partition/volume will be formatted and the selected drive letter will be assigned to it.
After closing Disk Management you might see some pop-up windows about formatting a partition before using it, and some AutoRun/AutoPlay windows. Just close these - most probably the tasks have already been accomplished.
"The system cannot find the file specified" error during drive initialization
Some users encounter the dreaded "The system cannot find the file specified" error while trying to initialize their new or existing disks. This message does not mean your disk is broken or unusable.
The error pops up mainly for older IDE (aka EIDE, ATA or PATA) drives that use jumper blocks to determine whether the drive is in slave or master mode. For example, when you have a slave drive inside a PC, you then put the drive into a IDE-to-USB drive conversion kit and connect it to another PC, you might just end up with "The system cannot find the file specified" error.
Disconnect the drive and set its jumper shunt (aka just "jumper") to Master or Single Drive position to get rid of this error. The jumper block guide is most probably on the drive's enclosure. If not, use the almighty Google (or its privacy-aware clone DuckDuckGo) to find instructions for the drive jumper placement.
SATA drives do not have jumper blocks anymore. The jumper shunts on IDE drives are mainly used for determining boot order: Windows always looks for boot record on master drive first.
Clearing read-only attributes for disks and volumes in Windows Vista, 7, 8, 8.1 and 10
In rare cases you might see an error message such as "The operation did not complete because the media is write-protected" or "The volume is read only" while trying to format or initialize a disk. This happens when someone or some program has marked the disk or volume/partition read-only.
In Windows XP, there is no easy solution available - Microsoft has provided no tools for this. You can boot from Windows Vista or later installation DVD article to make the disk or volume writable. For example, you can create Windows 10 installation media, boot your XP computer from it, choose your keyboard layout, click Next, then choose Repair your computer, Troubleshoot, Advanced options and Command Prompt. Then follow the instructions below from the point where Command Prompt is already open.
In Windows Vista and newer, open elevated Command Prompt.
Windows Vista, 7 and 10 users should open Start menu using keyboard shortcut Ctrl+Esc and type "cmd" into Search box. Then right-click or tap and hold the result and select Run as administrator.
Windows 8 and 8.1 users can use keyboard shortcut Windows Key+X to open Quick Links menu and click Command Prompt (Admin) or Windows PowerShell (Admin). Command Prompt commands run in PowerShell as well.
Alternatively, open Apps search/Search everywhere using keyboard shortcut Windows Key+Q and type "cmd" into Search box. Then right-click Command Prompt and select Run as administrator from App bar.
User Account Control pops up. Click Continue or Yes.
In the Command Prompt window, type diskpart and press Enter key on keyboard.
The following steps are the same for both volumes and disks - just replace "volume" with "disk" if you need to clear the read-only attribute for a disk.
Type list volume and press Enter to see all available volumes. Find the troublesome one based on its drive letter/Ltr and size, type select volume <volume number> and press Enter. In this example, Volume 5 is write-protected and formatting failed. That's why its File System (the Fs column) is "RAW".
Now run attributes volume to verify the volume/partition really is marked read-only. Note that you should not include the volume's number this time, because it is already selected.
Read-only is set, so type attributes volume clear readonly and press Enter.
You can repeat the same process for other volumes on the disk, or the disk itself to be absolutely sure nothing is preventing you from formatting the volume/partition.
After you're done, type exit and press Enter. Then close Command Prompt and return to Disk Management. Right-click the volume you just made writable and select Format.
Renaming existing partitions or drives in Windows / File Explorer
You can set a descriptive volume label (or name) for any partition/volume. This includes partitions on hard drives, Solid State Drives, USB flash drives; but not read-only media such as CD-R or DVD-R. Easier distinguishing between existing partitions and drives might come in handy in advanced usage scenarios where you need to use the DiskPart utility for expanding or shrinking volumes, or when you need to use Command Prompt for Windows troubleshooting.
Renaming a drive or partition will not affect any data on it: you will not lose your files.
Open Windows / File Explorer (keyboard shortcut Windows Key+E), browse to Computer / This PC if needed and then right-click (or tap and hold on touch-screens) the drive or partition you want to rename. Select Properties from the menu, type a name into the topmost field of General tab and click OK.
For example, if you have a dual-boot computer, set the name of operating system (such as "Windows 7" or "Windows 8.1") here; if you have many USB sticks, set the manufacturer and size for a name (such as "Apacer 16GB"); or set a name for secondary drives or partitions (such as "Data" or "Videos").
You should only avoid the name "SYSTEM", as this is used for hidden boot and recovery environment partition since Windows Vista.
You might need to click Continue or Yes in User Account Control prompt after this.
Now the name is visible both in Windows / File Explorer and in command-line utilities, such as DiskPart. See, much easier to know which drive is which. 
Changing drive letters for existing drives/partitions in Windows
If you have mistakenly assigned a wrong drive letter to your new drive or you want to change a drive letter for some other drive, right-click on the disk/partition and select the Change Drive Letter and Paths command.
Please note that you cannot do this for the partition/drive that contains your current Windows installation - this would make your computer inoperable. Neither should you change drive letters for disks where you have programs or apps installed - the software might stop working properly after this.
Click Change.
Select a new available drive letter from Assign the following drive letter box and click OK.
You might see a warning that some programs relying on drive letters might not run correctly. Make sure you have no programs installed on the disk.
If there are no programs on the disk, click Yes. Otherwise, cancel the operation by clicking No.
Converting a FAT/FAT32 volume/partition to an NTFS volume/partition without losing data in Windows
Some preformatted external hard disks and USB drives come with FAT32 file system to be compatible with smartphones and cameras, or older operating systems such as Windows 98 and Me. This is often no problem at all, but FAT32 file system is not good for large video and backup files. To be exact, you cannot create a file bigger than 4 gigabytes (GB) on a FAT32 volume: for example, creating a full DVD image or copying a large archive file for backup purposes is not possible.
Also, you cannot back up your Windows Vista, 7, 8, 8.1 or 10 computer to a FAT32 drive using the built-in Windows Backup program.
Luckily, you can convert any FAT partition to NTFS without losing data on the partition. This removes the 4GB file size limit, but also makes the drive incompatible with Windows 9x and Me computers. Well, 95, 98 and Me are really old anyway. 
Please remember that there is no easy way back - you cannot convert an NTFS volume back to FAT32 volume without losing data! There are such programs available, but most of these are not free of charge. One good example is AOMEI's free NTFS to FAT32 converter.
If you are sure you need NTFS, open Windows/File Explorer using keyboard shortcut Windows Key+E. Find the volume you need to convert and take note of its label and drive letter. Label appears before drive letter and a colon in brackets. In this example, volume label is "FATTY" and drive letter is "E:".
If there is no volume label specified, a general description such as "Local Disk" or "Removable Disk" is used instead.
Next, make sure there is at least 10 megabytes of free space on the disk before converting.
Next, close Windows Explorer or File Explorer, and all other open programs.
In Windows XP, use keyboard shortcut Windows Key+R to open Run dialog (or open Start menu and click Run). Type "cmd" and click OK.
In Windows Vista, 7 and 10, open Start menu by pressing Windows Key or by clicking Start button. Type "cmd" into Search box, right-click cmd.exe in search results and select Run as administrator. This will open so-called elevated Command Prompt (Command Processor).
In Windows 8 and 8.1, use keyboard shortcut Window Key+X to open Quick Links menu and click Command Prompt (Admin) or Windows PowerShell (Admin). Command Prompt commands run in PowerShell as well.
Alternatively, open Apps search/Search everywhere using keyboard shortcut Windows Key+Q and type "cmd" into Search box. Then right-click Command Prompt and click Run as administrator in App bar.
In Windows Vista and later, User Account Control will ask whether you want to allow the program make changes to your computer or not. Click Continue or Yes.
A black Command Prompt window will open. Type the following command: convert <drive letter>: /FS:NTFS /X
Replace <drive letter> with the letter of the drive you wrote down earlier ("E" in this example) and make sure you type a colon right after it.
Convert will check the current file system for the specified drive and ask for its volume label for added security. If the volume label you wrote down was not "Local Disk" or "Removable Disk", type the label now; else leave it empty. Press Enter key to confirm.
If the label was correct, Windows will check the disk for errors. This might take some time for larger partitions.
If the file system is fine, conversion will start. This might take up to half an hour, depending on the amount of data on the volume/partition. You will see Conversion complete after this.
If the disk check fails, you should run the following command in the same Command Prompt window: chkdsk <drive letter>: /F /X
Again, replace <drive letter> with the letter of the disk you want to convert, and make sure there is a colon right after it. Please note that the command cannot be completed on a system disk (where Windows is installed) or a drive that has open files - this will cause a message stating "Chkdsk cannot run because the volume is in use by another process. Would you like to schedule this volume to be checked the next time the system restarts? (Y/N)". Press y key and then press Enter to schedule a disk check, then restart your computer, wait for the disk check to complete and log back on. Then run convert <drive letter>: /FS:NTFS /X in Command Processor window again.
In case the disk is not locked and computer restart is not needed, repeat the command until you see "Windows has checked the file system and found no problems" message. Then run convert <drive letter>: /FS:NTFS /X again.
Deleting a volume/partition in Windows
If you want to delete a partition or a volume, please first make sure there is nothing important on it: this will also remove all files and folders on it. If you are not sure, create a proper backup first.
For obvious reasons, you cannot delete the partition where Windows is installed (while Windows is running). After all, how would poor Windows keep working if all its files were gone?
Right-click the partition or volume you want to remove and click Delete Partition (XP only) or Delete Volume.
A prompt appears, notifying that you will lose all data on the partition or volume. Click Yes if you are sure you want this to happen.
If you did accidentally delete a wrong volume and you never made a backup, you can use tools such as Partition Find and Mount or AOMEI Partition Assistant for recovery. Never reformat the partition you want to restore!
Shrinking a volume/partition in Windows Vista, 7, 8, 8.1 and 10
Sometimes you might need to repartition drives to have two or more partitions/volumes on one hard disk drive or SSD. Maybe you want a separate partition for your videos so that these files do not affect performance or stability of Windows when the video partition is full, or maybe you want to dedicate some SSD space for overprovisioning.
This process will not delete any files or folders.
To shrink a volume or partition, there must be some free space available on it and the partition must be formatted as NTFS. If you have a 250-gigabyte disk with around 70 gigabytes used of it, you can divide the disk into two 125-gigabyte partitions, or one partition with 75 gigabytes and one with 175 gigabytes disk space. The numbers are really up to you, but always leave some space for growth on both volumes. Take care while shrinking system partition/volume (the one where Windows is installed): leave at least 15-20 gigabytes of free space on the drive to have room for upgrades, temporary files, patches, etc.
For traditional hard disk drives, you should also defragment the disk before shrinking it, because if there are files scattered all over the disk, the partition size cannot be reduced even when there is plenty of free disk space available. Partitions and volumes must be allocated consecutively.
This does not apply to Solid State Drives (SSD), but using the TRIM command is recommended anyway. TRIM is supported in built-in defragmenter of Windows 7 and later.
Oh right, you will lose some disk space after dividing a disk into two or more partitions because some disk space will be used for describing the data, partitions and volumes available on it. Don't say I didn't warn you!
In Windows Vista and newer, this process is easy and all wizard-driven; in Windows XP this is not possible with tools Microsoft has provided, but you can use Resize and Move commands in free AOMEI Partition Assistant. This cool program also works with all newer versions of Windows, and it is able to overcome several other limitations of Disk Management. For example, it can move partitions/volumes - this is useful if you need to extend some partition.
Right-click on a NTFS-formatted volume or partition and choose Shrink Volume.
Windows will verify that there is sufficient adjacent free space on the volume. This takes from a few seconds to several minutes.
In the Enter the amount of space to shrink in MB box the free disk space available will be displayed. You can decrease it to your likings.
In this example, the volume has 30714 megabytes (or around 30 gigabytes) available, but I only need 10000 megabytes (about 10 gigabytes) disk space for the second partition.
After setting the numbers, click Shrink.
After the process completes, there will be two volumes on the Disk, one of them Unallocated (unpartitioned and unformatted). You can now create a new partition/volume there and format it.
Extending a partition or volume in Windows
You can enlarge an NTFS-formatted volume/partition if it has adjacent free, unformatted space on the same drive. This does not mean a formatted partition with free space on it, it means unformatted, unallocated disk space.
Extending is not supported on FAT/FAT32 partitions, or system partition/volume (the one where Windows is installed, usually drive letter C:). The latter problem, as well as problems with non-adjacent partitions/volumes can be easily tackled with free AOMEI Partition Assistant. I guess this is starting to sound like an ad, but I'm really not affiliated with them in any way.
If you do not need separate partitions on the same hard drive or SSD anymore, you must first move the contents of the unneeded partition elsewhere if you want to keep the files and then delete the unneeded partition. See the instructions above for deleting a non-system partition.
Again, extending a volume does not affect items already present on the volume you want to increase.
In Windows XP, you need to use the command-line DiskPart utility for this.
In Windows Vista, 7, 8/8.1 and 10, you can use the graphical and wizard-driven Disk Management console.
Right-click the NTFS volume or partition you want to make larger and select Extend Volume.
Extend Volume Wizard opens. Click Next.
The Select the amount of space in MB box on the bottom right contains the largest available disk space in megabytes for volume extension. You can decrease the size if you want to preserve some space for another volume/partition.
After setting your preferred number, click Next.
Click Finish to complete the volume expansion.
In Windows XP, use keyboard shortcut Windows Key+R to open Run dialog (or open Start menu and click Run). Type "cmd" and click OK.
Type diskpart and press Enter key to launch the program.
Run list volume command to find the volume/partition you want to extend. In the example below it is Volume 2 with drive letter F (the Ltr column). A volume that has "System" written in Info column is the partition where Windows is installed. Note the number of volume you want to resize.
Type select volume <volume number>, the digit being the number of volume you want to extend. In this case, the command is select volume 2.
As usual, press Enter key to launch the commands.
Type extend to add all available free space to the size of the partition. To specify an exact size for growth, type extend size=<number of megabytes to add>.
After the operation is complete, type exit and press Enter to quit DiskPart.
In the example below, list volume command was repeated to verify the volume size change.