Windows 8.1 automatically assigns a drive letter to each hard drive, or partition therein, in the order they are recognized. Assignment begins with the letter "C" for the Windows boot partition and increments alphabetically. Primary partitions on internal hard drives are recognized before optical drives, so if you install a second hard drive, the optical drive's letter could change. Likewise, Windows recognizes an external hard drive when it's attached, so changing the number of storage devices or the order in which they're attached can change the external hard drive's letter assignment. However, you can force Windows to use a designated drive letter, even when you disconnect and reconnect a drive.

Assigning Drive Letters

Pressing "Win-X" and selecting "Disk Management" opens the Disk Management snap-in that enables you to change drive letters. Right-clicking any partition and selecting "Change Drive Letter and Paths" gives you the option to change the drive letter to one that is currently available. If you want an external hard drive to always use the same drive letter, select a letter close to the bottom of the alphabetical list. When you later reconnect the drive, Windows automatically assigns the chosen drive letter, even if additional storage devices are connected.