Also try to get one without power management (cheaper anyway), I had a WD one that would shut off to save power and would never wake up until I unplugged it and plugged it back in. Tried a Seagate one with no issues.
I use a WD 1TB My Book A/V. It supposedly was designed for continuous use in fast transfer apps, although it may only be more expensive because it has an eSata interface as well as USB2. I got it when I was with D* and their EHDs need eSata. It doesn't power save.