My Tivo Series 2 TCD24004A just died a couple of weeks ago.
I'm looking to replace the hard drive.
Can I go to a 500GB hard drive?
I think you can go as large as 1TB, maybe even larger. The key is that the S2 uses IDE drives and those are hard to find (and expensive) in large sizes. The better route is to use a SATA drive but it requires a Tivo friendly adapter. Check the sticky thread in this forum for combinations of drives and adapters that work. I have used the adapter from WowParts in my S2 boxes.