Here is a solution from Microsoft.Please see if it is applicable to your problem (it is an article pertaining to external drives)
"For Windows Vista and Windows 7 users:
Click Start and type Device Manager in the Search box
Click Device Manager in the Control Panel list
Locate the Disk drives item in the list and click the small arrow to the right to expand the list of drives
If a disk drive is disabled you will see an icon with an arrow pointing down over the icon of the disk drive
If you see an icon with an arrow pointing down over the icon of the disk drive, double-click the icon and click the Enable Device button
Click Next on the Enabling a device dialog box, click Next, and then click Finish
Click Close and then close the Device Manager window
Note: If you don't see your disk drive in the listed drives, the drive may not have been detected by Windows.
Note: If you receive an error message when Enabling the disk drive, you may have a defective disk drive."
truenorth
P.S. Here is a link that goes into great length on dealing with issues of unrecognized external drives (of all kinds with the common factor being they are ALL USB connected).See if the above doesn't solve the issue. There are many possible reasons why the problem you are having can occur.
http://www.techspot.com/community/topics/troubleshoot-windows-wont-recognize-usb-hard-drive-fix-unassigned-drive-letter.122125/