can i use my DAT recorder to backup my pc via s/pdif optical?

It's not likely. SPDIF is for digital audio connections. It sounds to me like you are trying to backup your computer using a DAT audio recorder. DAT drivers for computer backup have a SCSI or IDE interface. It may be possible to decode/encode SPDIF for general data but it would probably be very slow. I don't think you're likely to find anyone doing this.

Here is an excerpt from the following URL: http://www.emu.com/products/darwin/faq.html

Can I back up Darwin to an Audio DAT?
No. You can, however, backup to SCSI DAT. SCSI DAT is a format that will write data via SCSI on to a 4mm DAT Tape.

Oh man, that's the next thing I want! I wanna be able to schedule backups of ALL of my data to run like in the middle of the night and stuff.... You get some really big capacity tapes now as well.... None of this is cheap, of course.