Sorry I've taken so long to respond, had an unexpected trip to DC over the weekend...
Nich: very good solution long term to prevent to issue of the OP.
Mr Man:
the movie and picture files in MotionEyeOS are kept bt default in
/home/ftp/sdcard/ camera name / date /
and
/home/ftp/storage/
the easiest way to delete all the mp4 & jpgs in those folders is:
/usr/bin/find /home/ftp -name *.thumb -delete
/usr/bin/find /home/ftp -name *.mp4 -delete
/usr/bin/find /home/ftp -name *.jpg -delete
This should at least clear enough space to allow it to start working again.
You can then modify the commands a bit to run in crontab to clear them regularly.
I would also set movies, preserve movies, custom, and set lifetime to 3 days. Ditto for still images.
Let us know if we can be of further help.