Sorry, should have mentioned, but assumed it was common knowledge, don’t use external USB or fire wire drives if it can be avoided. Use internal SATA’s or eSATA’s. If you have no space (in a laptop) preferably use eSATA or in the worst case fire wire. USB drives are too slow and drain too much CPU power from your system to work reliably.
Just for a frame of reference, sustained read/write speed and CPU load typically are:
USB: 20-25 MB/s @ 12% CPU load
Firewire: 25-30 MB/s @ 3% CPU load
SATA2: 50-70 MB/s @ 2% CPU load
What about your system specs?