Found searching net.
Only guessing but try this site
http://www.troublefixers.com/usb-pen-drive-detected-but-doesnt-show-up-in-my-computer/
Seen it but can't copy and paste it as it contains copyright on the site. Seems easy fix.
Bob
May 2012
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix usb pen drives.