I think brady might be on the right track. It may be a permissions relic - when sharing a drive or folder in Windows 7, the permissions attributes are propagated down through the file structure, so that all the file or folder objects are available at the correct level for the attached client user.
You may need to attach the drive to the original location where the drive was shared and using advanced sharing, reset the permissions to remove everybody for any shared access whatsoever, and leave the drive attached for a time so that all the permissions are erased.
I have a dual boot machine, with both drives shared for Windows 7 and XP with apparently identical machine names, workgroups, shares, users, passwords et cetera, but totally different behaviour on the network if the shared drives (internal and usb attached) are accessed when XP is the working OS than when it is booted under Windows 7. The one drive that always connects is the USB, whereas the internal drives are often difficult to connect to.