Anything with an eSATA connection will work.
However, without the PVR expander you will only be able to watch and record 1 channel at the same time (i.e. have to watch the channel you are recording) because the PVR expander has an extra tuner in it, allowing dual tuner capability. I find that to be quite useful, as often I record 2 channels and watch a recorded program at the same time.
Here's a cheap 2TB eSATA drive:
http://ncix.com/products/index.php?s...a&promoid=1223