After much reading of CP/M source code and stepping through the code in Joyce for DOS, I finally tracked down the vital byte to change so that drive C: is the default drive on my PCW. Now when the computer boots it looks on drive C: for PROFILE.SUB and doesn't care if there's no disk in drive A:. Although it still switches the drive motor on for some reason.

I thought I'd do a little speed comparison, so here is a programme I wrote a few years ago which loads a photo on the PCW, first being loaded from the floppy disk, and then from the SD card.