TivoS3 proactive measures?

I have 5 Tivo S3's w/LifeTimeService. What steps can i take to prepare for eventual hardrive failures?
1. I can upgrade the hardrives and remove orginals for storage.
2. Wondering can i buy a cheap TivoS3 off say ebay and use that hardive? Can you use a hardrive from another TivoS3 and just drop it in a S3 with a failed hardrive and it will work? Do you have to tweak the tivo to accept the used hardrive frm another used tivo?

Thanks