When you attach a USB mass-storage device to a computer running Windows XP or Windows Vista, the operating system assigns the device a drive letter – usually the first available letter. However, Windows reserves that drive letter for the USB device until your next reboot. So if you have a single hard drive (C:) and a CD\DVD drive (D:), and you attach a USB flash drive, Windows will assign the drive the first available letter (E:), but if you remove the flash drive and attach a USB hard drive, Windows will assign the drive the next available letter, which would be F: in this case. Windows won’t “give up” the flash drive’s E: until your next reboot.
This is all fine and dandy, unless your next available drive letter is a mapped network drive. Using the previous configuration as an example, let’s say that you have a network share mapped as F: on your system. You attach the flash drive, which gets assigned the letter E:. But then you attach the USB hard drive and… you don’t get a drive letter! Windows will show that the device is connected to your computer, but the device simply never shows up in Windows Explorer.
Don’t panic – nothing’s wrong with your system. Windows is just being stupid. There are two ways to fix this problem: you can reboot – which easily fixes the problem but isn’t always what you want to do. Or you can change the drive letter assigned to the USB device via Disk Management. To do this, simply click on Start > Run and type “compmgmt.msc” (without quotes) into the box, then click “OK” or press enter. In the console tree, click the “Disk Management” applet, then look in the right-hand pane. You should see your USB drive listed with the “unavailable” drive letter. Just right-click on the device and select “Change drive letter or path” and change the device’s drive letter to something available on your system. Your USB device should then immediately become available in Windows Explorer!