Compute Module 4

Perhaps these days it’s best to start at :0 and just count up, with no more removable/fixed distinction?

That is probably not advisable with the C based version of ADFS now being used in some machine builds.

FileCore documentation states 4-7 are Hard Disc and 0-3 are Floppies. Where things have muddied (ignoring legacy IDEFS etc that use 0-3 for partitioning) is with SDFS and SCSIFS, where fixed/removable aren’t so clearly defined or can even be determined by the OS.

For the sake of sanity, following the documentation until MBR/GPT partitioning support is added would be advisable; at which point drive numbers should become academic as they’ll need replacing with GUID’s or a new user-friendly scheme.

except if you stumble upon an NEC PC98 style machine where A: is the harddrive.

Or somebody has been “adventurous” with SUBST.

at which point drive numbers should become academic

There’s probably a painful amount of stuff kicking around that addresses media like :0.$ or :4.$.

as they’ll need replacing with GUID’s or a new user-friendly scheme.

User friendly, absolutely.
Because nothing says painful quite like having to type SDFS::6B29FC40-CA47-1067-B31D-00DD010662DA.$ ;)

stuff kicking around that addresses media like :0.$ or :4.$.

or even SDFS::CMOS which is designed to match SDFS::0.$.CMOS or SDFS::4.$.CMOS (I think)