Actually I did it twice. Once to add a drive, once to replace both drives. The second time was because I was having problems that were, according to common wisdom, symptoms of drive failure. Replacing both drives and boosting to 240G, didn't solve the symptoms.

That was a bummer. I found what the problem was totally by accedent. While searching for "lifetime subscription" I came across a thread about power supplies. (Tivo Comm. Forums) It seems the power supply of some tivos just didn't have the guts to survive the long term extra load of a second drive. Someone who had a similar problem with a PC recognized the problem and brought it to everyone's attention. Turns out that an exact replacment supply only costs $4.99. (the shipping was more than that). Solved my problems . I could have saved my self... Oh Well.

PS search terms are a pain. I would have found that thread earlier, except that I was searching for "Frozen video", while that thread referred to the problem exclusively as "stopped Video"