Hmmmm...... Interesting.....
My partition scheme does appear to be APS rather than GUID so firmware updates probably won't work.
On the other hand, current firmware is the latest - MM11.0055B08 (EFI 1.1) and 13f4 (SMC 1.0) and Apple probably won't update for this older Mac Mini.
Software updates and the machine work fine so I don't think I'm going to worry about it. If I ever need to reinstall OSX on this drive I'll just image the Data partition, repartition as GUID, install OSX on the primary partition and restore the image to the secondary one.
A most interesting little detour around the Mac ecosystem!
Thanks
innuendo and the rest of you!
Mac
PS: Linux (and my long-time fave is MEPIS -
https://www.mepis.org/ ) is easier to manage and has fewer odd little surprises like this)