For future readers …
I think (hope 
) I have solved this by just getting a bigger (124GB) SD card for my RPi:
I was getting this problem on large data transfers (for example when Shotwell rewrote the metadata to all my many photos due to me forgetting to mount the external disk)
I suspected that:
- During the data transfer the SD card of the RPi was getting full.
- Since this ocurred as the data was being transferred making more space (see my methods for this at end of this post) on the SD card after this was too late to be any help
The SD card I was using was an 8GB one that was way bigger than the Core Appliance needed, but I thought that perhaps a bigger one would solve my problem
So I got a 124GB SD card (around €15) and cloned the (working) SD disk to this
Just now, I had the Shotwell problem again, but the RPi coped with the large data transfer 
Woohoo!
ME@localhost:~$ df -H
Filesystem Size Used Avail Use% Mounted on
udev 404M 0 404M 0% /dev
tmpfs 96M 11M 85M 12% /run
/dev/mmcblk0p2 126G 8.5G 111G 8% /writable
/dev/loop0 52M 52M 0 100% /
/dev/loop1 195M 195M 0 100% /lib/modules
tmpfs 476M 4.1k 476M 1% /etc/fstab
tmpfs 476M 0 476M 0% /dev/shm
tmpfs 5.3M 0 5.3M 0% /run/lock
tmpfs 476M 0 476M 0% /sys/fs/cgroup
tmpfs 476M 0 476M 0% /var/lib/sudo
tmpfs 476M 0 476M 0% /mnt
tmpfs 476M 0 476M 0% /media
tmpfs 476M 164k 475M 1% /tmp
/dev/loop2 325M 325M 0 100% /snap/nextcloud/28552
/dev/loop3 12M 12M 0 100% /snap/pi/74
/dev/loop4 320M 320M 0 100% /snap/nextcloud/28579
/dev/loop5 30M 30M 0 100% /snap/snapd/13269
/dev/loop6 12M 12M 0 100% /snap/pi/98
/dev/loop7 320M 320M 0 100% /snap/nextcloud/28592
/dev/loop8 30M 30M 0 100% /snap/snapd/13171
/dev/loop11 30M 30M 0 100% /snap/snapd/13643
/dev/loop10 194M 194M 0 100% /snap/pi-kernel/345
/dev/loop9 52M 52M 0 100% /snap/core18/1888
/dev/loop12 201M 201M 0 100% /snap/pi-kernel/185
/dev/sda1 984G 584G 351G 63% /media/Work
/dev/mmcblk0p1 265M 128M 137M 49% /boot/uboot
tmpfs 96M 0 96M 0% /run/user/1000
The /dev/mmcblk0p2 126G 8.5G 111G 8% /writable line seems to show that if the card was only the previous 8GB one then I would have had a problem
I then ran
sudo nextcloud.occ versions:cleanup
which got it down to
/dev/mmcblk0p2 126G 3.5G 116G 3% /writable
But
sudo -i
cd /var/snap/nextcloud/common/nextcloud/data/Leigh/files\_versions
rm -R *
and
sudo nextcloud.occ trashbin:cleanup --all-users
Didnt make any further difference, i.e. it was still at
/dev/mmcblk0p2 126G 3.5G 116G 3% /writable
Update 09/11/21
Alll has been working perfectly with the new card
I just changed the tags on a lot of photos with Shotwell, precipitating a 3 GB upload to the Nextcloud Box and all went well (which would not have happened before I got the new card)
However, df -H now gives 12G used on the card:
/dev/mmcblk0p2 126G 12G 108G 11% /writable
So the card has gradually been filling up
sudo nextcloud.occ versions:cleanup
sudo nextcloud.occ trashbin:cleanup --all-users
got it back down to 3.6 G (about half with each command)
/dev/mmcblk0p2 126G 3.6G 116G 3% /writable
But shouldnt it automatically clean out these things?