That's a really good qustion. I never have luck recording with mine - tried three different usb thumb drives and it seems to hate them all. It always locks up and then reboots. Once I recorded the History Channel with my 320 WD hard drive, but it has never worked again. I've checked all inside connections, tried different firmware and even formatted the application area to no avail.
Iceberg has the same hard drive as I do and he doesn't have any problems recording, so it isn't likely my drive.
I would think something with a fast write speed that wasn't over 1TB would be the way to go. I've always used WD drives in every system I've built and haven't had one fail yet. The current 640GB drive in my PC is the fastest quietest drive I've ever had.