I said I wasn't going to stop the PCW project just yet, so here's a bit more progress. I got multiple logical drives working on the SD card this evening. I should probably change the message — drives C, D, E and F are all on the same card:

I started with 32Mb partitions, but CP/M was only letting me create two drives before running out of memory, so I reduced the size to 16Mb, because I though having multiple drives might be more useful. Especially as I've started to add the ability to map more partitions on the SD Card to the four drives. The FID file keeps a record of which partitions are mapped where — I just need to write a user programme to switch the partitions in the drives now.