TiVo Premiere losing storage capacity

My TiVo Premiere (2 tuner, 500 GB) has suddenly started losing storage capacity. I keep my HDD full and stuff just gets deleted when it needs space. The capacity meter was always in the high 90s. But about a week ago, the capacity meter suddenly was at about 83% and it was still bumping old stuff off to record new stuff rather than filling back up to 95+%. Is it possible the HDD is going bad and therefore the TiVo is quarantining bad sectors and that's why it's not filling up to capacity? Or is there something else that's happening.



And if the HDD is going bad, how difficult is it to put a larger HDD in there? I did it on a couple Series 2s about 10 years ago, but haven't had any experience doing it on the HD models.