So does 6412 III have to be on to record or just plugged in? I called Comcast to ask they said it just needs to be plugged in, but last night when it was time to record, i had my eye on it, the recording light didn't turn on. When i turned on the TV it said 100% full, only had 45mins of Discovery HD in it.

So will it work or will this always happen? should I just leave it on...wouldn't this over heat the unit?

It will power itself on when it's time to record the scheduled program. You have a box that suffers from the 100% full bug apparently, do a search on here about it.