In Windows operating system especially Windows XP, when plug in external USB or FireWire mass storage device such as USB key, flash drive, portable harddrive, the drive is not been assigned a drive letter by the system, hence the USB drive or FireWire drive is not showing in Explorer and no AutoPlay or AutoRun window pops up, although the device has been detected, in Safely Remove Hardware (show device but with no drive letter) wizard. The cause is probably due to ‘stupid’ behavior of XP where it won’t find an available letter from free letters pool when the USB or FireWire external drive which has previously mounted and assigned a drive letter is been inserted to computer USB or FireWire port again, but that previously allocated drive letter has been taken up and used by another storage devices. Another possible reason is all 26 letters in alphabet has been used, but this situation is rare and unlikely.
There is a solution to USB or FireWire drive not showing in system error. However, the disconnect conflicted storage device to free up the drive letter fix may not practicable or usable by some, such as computer which are on a network or system running data transfer activity to all its drive assignments continuously flow without stopping, and thus disconnect or reassigning any mapped drives, networked drives, removable drives or other storage drives is not an option.
In this situation, there is another workaround hack to fix the no USB or FireWire drive issue. The workaround fix relies on the registry tweak below to change the drive letter that has been previously assigned to the mounted USB or FireWire portable mobile flash or hard disk drive. Or if users wish, can opt to delete any reference to the detected drives by the device so that when the storage device is plugged in again, the whole drive letter assignment will be start from fresh, and clean.
- Login to Windows as an Administrator.
- Open Registry Editor by typing regedit (or regedt32.exe in older Windows prior to XP) in Run command of Start Menu and then press Enter.
- Navigate to the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices
- Exclude \DosDevice\A: and \DosDevice\B: which normally reserved for floppy disk drives.
- Exclude \DosDevice\C: which normally is the system root drive.
- If you have more than one fixed hard disk drives, then any drive letters that are using by them is not possible linked again, UNLESS the additional hard disk drive is added AFTER you first use the USB or FireWire device that now unable to show.
- Same case with CD-ROM or DVD-ROM optical drive as above reasoning.
- If you have inserted and mounted a lot of thumbdrives, USB flask drive, or external hard disks before, you will likely still see a lot of remaining registry values that you won’t know which is which. In this case, double click on each remaining registry key values to view its binary data. Inside the binary data, there will be trace of the name of the device that this registry key represents.
Simply delete the devices registry keys instead of renaming it. Deletion is helpful is users really can’t find the the registry value for the drive letter that having problem. But make sure that you do a backup for the registry branch by exporting MountedDevices key, as mistake may cause your system unbootable. The delete all possible USB drives, FireWire drives and external drives which have drive letter temporarily only when inserted in the system, and DO NOT delete any of the fixed hard disk drives or CD/DVD optical drives registry values.
from: www.mydigitallife.info
Other source says...
When a removable drive (USB flash drive, flash card reader, portable hard drive) is attached for the first time, Windows mounts it to the first available 'local' drive letter. If there is a network share on this letter, Windows XP will use it anyway for the new USB drive because since XP network shares are specific to the current user and not visible in the context of the system where the letter is assigned. The USB drive then appears to be invisible. This is fixed by SP3 in most situations.
You can change the letter assignments in the Windows Disk Management Console with a lot of mouse clicks but you have to do it again for every new device.
And, for USB devices that have no serial number (in violation of the USB standards) you have to do it too when you attach it to a different USB port.from: www.uwe-sieber.de/usbdlm_e.html
Here a simple tool (DriveLetterView by NirSoft)
