Fwiw, a 1GB MD would be Hi-MD, a different media in the same form factor. I don't actually know if the Hi-MD players support NetMD transfer or use their own protocol, but I guess I can think of a way you could find out!

Yup, mine was NetMD. Didn’t realise that worked with the older discs too, never had a regular minidisc.

Yep! "NetMD" was just a new logical format on the same media, plus USB connectivity and ATRAC version 3. Hi-MD was the one that reformulated the media for 1GB total storage. Those discs don't work in older players, but older discs do work in a Hi-MD player.