I had a similar type problem, my SD/MMC converter drive, or a camera used to pop up in "My Computer" as a drive (G:) and then after I mapped a Network Drive and used letter (G:) suddenly next time I connected my SD/MMC converter drive, or a camera it did not pop up in "My Computer" at all, although there was a beep noise to indicate that the USB was connected.
Here is what I've done:
1. (Ensure that you have connected your USB device ),then Got To : "Control Panel" -> "Administrative Tools" -> "Computer Management"
2.On the left hand side of the "Computer Management Explorer" select "Disk Management" you should see your removable storage device in the list with the same letter as your mapped network drive.
3.Do right button on your removable drive and choose "Change Drive Letter or Paths..." and assign different letter.
That is all, go back to "My Computer" and the drive should be there. Next time you connect it, it will pop-up with a newly assigned letter.
Or maybe you can just map you network drive with a different letter :)
That's what worked for me I don't know if its the same case with you ...
P.S Good Luck