We get the question every day: what’s the difference between an “Add” kit and a “Replace” kit?
The basic bottom line is this: Replace kits are meant to go in your unit in place of the hard drive(s) in there now. They have the right TiVo OS on them for your exact model of TiVo, and when you pop one in and start back up, you’re in Guided Setup, just like when you first installed your unit (except maybe a much newer version of Guided Setup – we try to keep sending out the latest stable OS).
Add kits, on the other hand, are formatted entirely differently, and usually ship with additional parts. Add kits are essentially filled with extra storage partitions. Your TiVo’s factory drive sees the partitions, “marries” to them, and starts loading them up.
More info is here:
The key is to know which you want, and to order appropriately. One really isn’t a substitute for the other. But both units are considered “upgrade” kits since either way you go, in most cases you get far more storage than you had before.