When using external drives, Lightroom doesn’t change the drive letters, but Windows often does. That can confuse Lightroom, requiring you to relink missing files on a regular basis. Leaving the drives plugged in to your computer, or always reattaching them in the same order can help avoid the drive letter changing.
If you want to set the drive letter a little more permanently, or reset them if they change, you can go into Windows Disk Management and assign a specific drive letter. You’ll need to be logged in as an Administrator. On Windows 7, go to Start menu > Control Panel > Administrative Tools > Computer Management. In either case, Disk Management will then be listed in the left hand panel under Storage. Selecting this will connect to the Virtual Disk Service and will display the drives seen in the main panel.
Find the relevant drive in the list, right-click and there’s an option to Change Drive Letter and Paths. Be very careful to ensure you have the correct drive! Selecting Change will display a list of available letters to select from. Selecting a letter outside the range Windows would usually assign automatically helps to reduce the possibility of it changing, so a letter from the latter half of the alphabet is a good choice. When you’ve finished, press OK, and select Yes when prompted to confirm the drive letter change.