This tip from a recent WindowsSecrets LangaList column might help:
_‘Windows can’t see an external USB drive,’ I have had similar problems with my multiple USB drives and Windows 7.
"I found that the solution was to go into Control Panel, Administrative Tools, Computer Management, Storage, and then Disk Management.
"Once I got there, my USB drive showed up in the right-lower panel — but I had to scroll down to see it. It did not have a drive letter.
“From there, it was a simple task to right-click the USB drive image, click Change Drive Letter and Paths, and assign a path. Like magic, the system recognized the external drive.”_
hth
