I ran into a problem with a client today where we were trying to add some USB external storage for added onsite backup storage. The problem was, it never seemed to show more than 1 of the 2 external drives.

I tried the 2nd drive that wouldn’t show up on other machines, and it worked fine.

Turns out the problem exists in more recent version of windows. This happened on Server 2012. If 2 devices are connected, it will only assign 1 of them a drive letter if they have identical signatures.

This explains the fix using diskmgmt:

http://wdc.custhelp.com/app/answers/detail/a_id/6571/~/only-one-drive-is-seen-when-connecting-two-wd-external-drives-of-the-same-size