Method one could be used to allocate a drive letter to each USB drive. Method 2 will remove all previous drive letters for a fresh start.
{1}
Try rebooting the computer without any USB devices attached. Then plug in the USB drive, right click on My Computer, select manage, and then click ?Disk Management.? If the drive is not present in the Disk Management window or if you are not able to access it, then the drive may have failed. In this window you should see all of your connected physical drives, their format, if they are healthy, and the drive letter.
Change the drive letter of the drive. Right-click on the drive in the list, and from the resulting menu select "Change Drive Letters and Paths"
Click on Change so you can change the drive letter. Click Change. Select a new drive letter from the drop down list, preferably a letter that is not normally used for this drive.
Click Yes on the confirmation screen and you are done.
{2}
Write down all fixed Disk and CD/DVD drive letters so as not to delete them. DO NOT delete any of the fixed hard disk drives or CD/DVD optical drives registry values.
1. Login with Administrative privileges.
2. Disconnect all USB or FireWire storage devices.
3. Open Registry Editor by typing regedt32 in the Run box and navigate to the following registry key: HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices. Right click on MountedDevices and select Export. Save this file as you may need it in case something goes wrong. Save it as Devices.reg, it should default to My Documents.
4. Under HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices there will be registry keys with the name that resembles the format of \DosDevice\X: where X can be any alphabet letter which corresponding to your physical system drive letter.
5. Delete all possible USB drives, FireWire drives and external drives which have a drive letter.
DO NOT delete any of the fixed Hard Disk Drives or CD/DVD optical drives registry values.
Exclude \DosDevice\A: which normally reserved for floppy disk drives.
