Lots of people have reported problems relating to USB file transfer. Here are some of the things that have worked for some people:
1) Use a USB port on the back of your computer, not the front.
2) In the Intercept's settings, make sure USB debugging is OFF.
3) Once you connect phone and computer, you are pulling down the notifications, hitting the USB one, and then selecting "Mount" right?
4) When you are done with the connection, you are using "Safely Remove Hardware and Eject Media" to disconnect the phone, right? This is for Win7, Vista, and maybe XP.
5) If you move/copy files and something hangs, disconnect the phone, eject it from your computer, turn the phone off, then turn it on again and start over.
6) If you get a hang on a large amount of data, move/copy only a few files at a time. If you hang on really large files, I can't help you as I haven't figured out how to fix that yet.
If you're totally stuck and nothing works, you can move/copy files via FTP. Search the Market for file managers that have that capability.