The reason you can't see the USB device in My Computer is (almost certainly) that you have a network drive mapped to the first drive letter that is not in use by a local device (C, D, E, etc.). It's an easy fix:
- Right-click on My Computer and choose Manage
- Click on Disk Management
- Look for your USB drive's name in the volume list (upper half on the right) or disk list (bottom half on the right)
- Right-click on your USB drive's entry in the list
- Click on "Change drive letter and paths"
- Click "Change" in the window that opens up
- Choose a drive letter that is not in use by a local device OR a network device
- Click "OK" all the way back out of the dialogs
Give the computer a moment and then check to see if it shows up in Explorer now.
The reason for this is that for whatever reason, Windows isn't smart enough to realize that the letters network drives are mapped to are in use and so it shouldn't assign them to local devices. Once you've done this process once on a given computer, it should pick up whatever drive letter you assigned it each time you plug it in.