I think the issue is the PVR doesn't use a typical file system, they do a bare write to the drive. In that case, because if differences between drives (speeds, cache, etc), they only allow "approved" units.
I played with the HDD in my 9200 and 9242, and like I mentioned above, I wouldn't do it again. One went down the yellow brick road pretty far, and it luckily self healed itself (it was stuck in a drive error detection screen for 1/2 day). And, once the drive is mounted in the PVR, they muck up some base drive bios settings that makes it no longer detectable by a PC's bios (the sleep mode or something).
So, like I said, wouldn't recommend it if your PVR is functioning. If the drive punks out, then play away, but I wouldn't risk it on a working PVR again. It seems pretty fickle.