Alternatively called a device letter, a drive letter is a single alphabetic character A through Z that is assigned to a physical computer drive or drive partition. For example, a computer with a 3 1/2" floppy diskette drive has a drive letter of A: assigned to the drive. Computers containing a hard drive always have that default hard drive assigned to a C: drive letter, and the CD-ROM or another disc drive is the last drive letter (e.g., D:).
In the above picture, you can see three different drive letters (C:, E:, and F:). Additional drives may be added when you connect a removable drive such as a USB flash drive.
How to view the available drives
How is a flash drive, smartphone, or another drive assigned a drive letter?
In Windows, when a flash drive, smartphone, or another drive is connected to the computer, it is assigned to the last drive letter. For example, if the last drive letter is "D:," when a new drive is connected, it's automatically assigned as "E:" drive until it's disconnected.
Why are all possible drive letters not shown?
By default, Windows is designed not to show any drive letter unless the drive was accessed or is accessible.
If your computer has a card reader, it may list the drives as available for each of the available slots. However, none of these drives are accessible until a card is inserted into one of the slots.
To identify a USB drive letter, disconnect the drive and then while viewing the available drives connect the drive again and watch for a new drive to appear.
Why am I missing a drive letter?
If a drive becomes disconnected or has problems, it's removed from the drives list. When you reconnect the drive, as long as no other drives were connected to the computer, it uses the same letter. However, if another drive is connected before the previously disconnected drive is reconnected, it becomes the last drive letter.
How do I identify what letter is assigned to a drive?
It can be confusing sometimes when you are trying to determine what drive or partition is associated with what letter. In Windows, you can usually identify the drive and its associated partitions with Disk Management. Other drives listed could include a connected USB thumb drive, smartphone, digital camera, MP3 player, etc.