Just using the standard DVCPRO HD codecs provided by Apple in the final cut suite available after 5.0 i beleive. 5.0.1 has them installed if i remember correctly.
Ive edited off of USB drives at 5400rpm, hell, ive even edited off of bus powered USB drives, you dont need any sort of special hook ups to edit,
BUT
if you are going to be editing off of external drives, it is highly recommended to use faster bus ports, like if you are on a Macbook Pro , FW800, or if on a desktop eSATA or FW800 preferred.