I now have an external USB HD myself and after formatting it on Windows XP and using it on a Linux machine the XP machine wouldn't access the drive again. The Windows 7 Starter Edition Netbook found it no problem. The XP Machine would find it with defrag for example but not allow any use of the drive.
Here is how I cured it (on XP):
Control Panel
Administrative tools
Computer Management
Disk Management
Right Click on drive name choose Change Drive Letter and Paths.
Assign it an unused drive letter.
That did it for me.