If you are using windows, make sure you open up the Computer Management console (Right Click on My Computer and select Manage Computer) and go to the "Disk Management" selection in the left pane. From there, you should see all the attached devices (CD/DVD Drives, Hard Drives, USB Drives and Mapped (Network) drives.) You will want to look in My Computer to see what is in use, and if the iPad is trying to use a drive letter in use, you will need to change it to one that is not in use.
It is possible the iPad is trying to utilize a drive letter that is already in use by a network drive.
A: is a floppy device
b: is a floppy device
c: is your main HD
d: is generally either a second internal HD or CD/DVD drive.
e: is generally either a second internal HD or CD/DVD drive.
f: will be the first network drive (generally).
There is more help on this issue on Apple's support forums as well - especially when it comes to MAC's and this issue. I have only experienced the issue on a PC.