It's very possible, given the age of the technology involved that either deck is toast. The fact that you can see good picture in the viewfinder means that the Hi8 heads are probably OK but everything after that in the chain is suspect (jacks, cables, etc). Swap all cables and try again till you find what is malfunctioning.
My suggestion would be to get ahold of a Digital8 camera or deck. It was a hybrid format from the early days of DV that played Hi8 tapes but had a FireWire DV out and was controllable from FCP.
However, neither of these methods (yours or mine) give you timecoded recapturable media, which would be necessary in the case that you lose your data. If you have tons of Hi8 and DV you might best be served by dubbing the Hi8 to DV and then capturing from that with it's somewhat useable timecode that can be logged/captured and if need be, recaptured.
Alternatively:
You could just capture all your SD NTSC tapes in, whole or in more manageable 10 minute chunks, and back them up in many places and make those your new masters. A rough estimate of space required for storing SD ProRes (NTSC 720x480/2ch 48kHz 16bit) is about 20GB/hr.
ak
Sleeplings, AWAKE!