Settings/System Settings/USB Mode.
There are two modes and your computer can only see one at a time.
Try MSC and try MTP. Your files should show up under one or the other.
Auto Detect chooses MTP if it sees Windows Media Player 10 or above on your computer, MSC if not. If you sent stuff over from different computers, Auto Detect may have used different modes.
Easiest thing to do is to take everything off, set a permanent mode (MSC is less troublesome) and put everything back on.
You only need MTP if you are using files that have Digital Rights Misery, like library books that expire or music from a subscription service (Rhapsody).
