Alternatively referred to as 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. All computers with a hard drive will 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 is automatically assigned to as the "E:" drive until it is disconnected.
Why are all possible drive letters not shown?
By default, Windows is designed not to show any drive letter unless the drive is using that drive letter.
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 will be accessible until a card is inserted into one of the slots.
A good method of identifying a drive letter of a USB drive is to disconnect the drive and then while viewing the available drives connect the drive and watch for a new drive to appear.
Why am I missing a drive letter?
If a drive becomes disconnected or has problems, it is removed from the drives list. If it is not the last drive when it is detected again, it will use the same letter. However, if it is the last drive and another drive is connected and takes over the drive letter when that drive is detected again, it will become the last drive letter.
How do I identify what drive is using a drive letter?
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 by using Disk Management. Other drives listed could include a connected USB thumb drive, smartphone, digital camera, MP3 player, etc.