Hi Luzie, thanks for the welcome! And thanks for the suggestions too.
The When I realised that snapshots were not working I tried several times and the file system of the SD card became corrupted. I got some garbled text and "COPY COPY COPY COPY COPY COPY COPY COPY COPY COPY COPY COPY" appearing on the ESXdos startup screen when I powered the Omni up, so I reformatted the card with the official SD formatting utility and replaced the files from a backup.
I do have a tmp folder in the root directory.
I tried reverting to the old NMI handler from within the new one and that seemed to have some success at saving snapshots but not all... I saved a snapshot of an old 16k game successfully but seemed to have a problem saving a snaphot of a 128 game, I don't know if that's significant.
After trying to save another snapshot with the new NMI I got a red band across the centre of the screen with "ESXDOS ERROR 19" on it. I reset the Omni and am now getting an error when it tries to load esxdos.xyx on startup.
I'll reformat the card again, check it for errors and load the files back on. The card has been working fine with the old nmi handler for several months.