While some software / firmware / incompatibility issue may still exist (will have to wait on Tablo to check that out), from my view, its starting to either sound like a bad Tablo (ie, internal USB driver failing) or its still possible its a bad Tablo power supply.
If you happen to have a compatible power supply, I would try that first, if not, Tablo has been known to send replacements out but I'm sure they will want to exhaust possible software issues first.
Another option, if you happen to have a powered USB hub available you could put that between the Tablo and HDD to get the Tablo out of the middle of supplying power to the HDD. If the Tablo power supply is borderline, this might be enough to keep it running. The fix would still be to replace the Tablo supply but its a step that could be taken for testing purposes.
Hope some of this helps, I know its frustrating. My original Tablo had a bad tuner in it, I was sure I had made a mistake, but Tablo did come through and replace it under warranty and since then (almost 2 years now), it has been 99% issue free.