G'day mate...
Before you buy your WD ext HD... look at the WD My Book Studio Edition 1TB for Mac. Has 2 Firewire 400 800, eSATA and USB 2.0 it is designed to work with Mac... and it does very well. I run FC Pro.. and use this HD as my 'Scatch Disk" via Firewire 800... I don't use the WD HD as back-up as I have Time Capsule 1TB.. that back's up my iMac and data on the WD HD.. best of both worlds...
Good luck!