Why does the USB drive not show the letter?

A few days ago in helping others to repair the computer, my U disk plugged into the computer, the computer has identified the U disk, but open "My Computer" but can not see the corresponding letter. Go to Disk Management in Administrative Tools (enter Diskmgmt.msc in run), and you can see the disk without assigning a letter. Manually assign a disk letter G, but open "My Computer", still does not appear the U disk letter. Enter g:\ in the address bar, prompting that the file://g:/file cannot be found. If you format the USB drive on disk Management, the hint cannot be formatted and the volume is not enabled. If the right key in Disk Management just allocated g disk, select Resource management, the hint can not find g:\. But under the Windows Console (Command prompt, enter cmd in "Run") can enter the U disk, can operate normally (copy, delete, open file). Strangely enough, a removable hard drive can be seen with the appropriate letter.

Final Solution:

To the system directory C:\WINDOWS\system32\drivers see if there is a "sptd.sys" file. Remove it and reboot. Problem solved! Note: This "Sptd.sys" is not from Microsoft Windows.
Other possible causes:

1, the system has mapped network drive, resulting in the letter can not be assigned to U disk.

WORKAROUND: Disconnect the mapped network drive.

2, the letter is hidden.

Workaround: Enter the registry (Regedit.exe) and enter the "Heky-current-user\software\microsoft\windows\currentversion\policies\explorer" branch. Locate the key value "NOdrives" that exists for the path and delete it. Log off and log back in. (Not the NoDriveTypeAutoRun key value).

3, U disk problem.

Solution: Reformat the U disk. The best low-level format, to the Internet to find a U disk tool.

4, not really, that's the last resort: Re-clean installation of Windows xp!