connection interface, glatt?
usb? 1.0, 2.0? Firewire / 1394? eSATA?
That will make a lot of difference, though you can do little about it after the fact (you can't change the interface after the fact). Also, will you be only recording to the drive and playing from a different drive? (unlikely upon reflection.... poor question) For the capturing and editing you *may* find that it's more worthwhile to work (read/write/tempfiles/workfiles) on your system drive, (likely C
and then only transfer the finished product to an external drive.
Please keep us posted.