Does the /device/ appear as e.g. /dev/sdb when plugged in? Note that I'm not talking about the /filesystem/ here, or about individual /files/ on the filesystem.
Sorry, been busy for a few days with work.
I'm not using Linux, but I need to support it. I've only heard rumours that there is a RISC OS driver for Linux (or, more specifically, ADFS).
Under macOS, a RPi bootable RPi card (and disc image) has a FAT16 partition which macOS can mount. But, the RISC OS partition appears red and unmountable. Looking at the hex dump of the image I can see the RISC OS partition, as expected (in fact, it appears to be interleaved with the FAT16 partition). So I just need to be able to access an SD card in the same way as an image file.
I'm going to fire up my RPi next, booting into RISC OS, and format an SD card as ADFS, then see what macOS makes of it.
Note that the OP, although it sounds like he has the same issue as myself, is not trying to access a RISC OS partition.