A few questions.
1. I assume these are USB drives? If so, what USB version - 1.0? 1.1? 2.0? I find anything below 2.0 can cause this kind of lock-up problem. Remember it's not just your computer's USB version, but the drives themselves. Some aren't built to handle the full 480mb/s rate of USB 2.0.
2. Are you using a USB port hub? If so, make sure it's a powered hub, with controllers for EVERY expansion port, not just a single controller handling all of them. I've found it vital to have this if I'm to prevent conflicts between devices (and I have a couple of high-use USB devices).
3. If you have more than one high-speed USB device on the same port (i.e. in a hub connecting to a single port), you need to change that. I found that my external USB drive (a Maxtor) would hang up if it was on the same hub as my wireless keyboard and mouse (I touch-type at 100+ wpm, and the signals were clashing). I solved the problem by plugging the hard disk into its own dedicated USB port on my computer, and leaving my keyboard and mouse on the hub plugged into another port.
Finally, check to see what generation of controller your PC is using. Most modern systems control each USB port independently, but some earlier ones (and some el cheapo modern ones) simply daisy-chain the USB ports in series. This means that a heavy load on one affects those downstream of it in the daisy-chain. Worth investigating.
Hope this helps.
