I don't know about the MDS-501, but My Sony MZ-R30 has optical input only, not output, and I seem to remember that's the usual rule with the portable recorders.

It wouldn't mention computers - optical links were supposed to be for transferring between MD recorders, to an amplifier with a digital input, or for recording from a CD player with optical digital output. Sound cards with optical or coax S/PDIF emerged only after Sony's minidiscs came on the scene, but I don't think Sony liked the implications of being able to copy your audio to a computer where further copies could proliferate!

I still have my MD deck and MZ-R30, but I never use them now - the quick and unrestricted USB transfer to computer from the Edirol R-09 has left MD way behind.

Anahata