I have an ancient relic minidisk recorder with optical input. I was wanting to record some drum tracks onto it from my PC.

Q-can it be done? How can it be done if it can be done?
Not sure why you'd need to do that but you need toslink cable and both output and receiving device have to be lightpipe spdif:

Usually lightpipe interface will have a digital out which you have to select and the right clocking rate needs to be set on the minidisc, mine had a 44.1 and 48khz slider switch I think...

Then it should work just like audio.