I was going to report a hard drive fault with my TiVo this morning (recordings are virtually unwatchable at present, live TV is fine) but have found the little fella is stuck in a constant reboot loop this morning. I had the software update last week so don't think it's related to that.
I unplugged the box from the mains for a minute or so, and the box seemed to settle down, although I got a 501 message and it appears all my recordings and series links (and user preferences) had all been reset. Not a problem as there were only a few recordings on there, not life or death.
Managed to set a recording for tonight (for what it's worth) and powered off. Now a few minutes later the box is back in the reboot cycle.
Help ! I assume I'll need a box swap out for the hard drive fault anyway.