USB drives are an essential backup tool for road warriors, but occasionally Windows Vista tricks you by not assigning them their usual customary drive letter. I regularly use SyncBack, a perennial Lifehacker reader favourite for backup and file syncing, and it doesn't work if drive E: suddenly shows up as drive F:.
Fortunately, you can remap drive assignments in Vista, though it's a well-hidden option. Right-click on the Computer icon on your desktop or start menu, then click Manage (and go through all the User Account Control confirmation malarkey if you have it switched on). Expand Storage at the right, and select Disk Management. Right click and select Change Drive Letter and Paths, then select Change. Choose the letter you want and click OK. You may get an additional warning or two saying the drive letter is unavailable until you reboot, but press on regardless -- I've found it nearly always works. If you do have multiple drives or devices plugged in, you might also need to reshuffle the existing mappings to make the one you need available. For a more permanent method of assigning a letter to your USB drive, check out this method for creating shortcuts.