I went a different route with this and used D.O. volumes to add additional space
Mount new storage on Digital Ocean
Delete all UFW rules, except for SSH
Create a filesystem for an external volume.
Create a mount point for your volume:
mkdir -p /mnt/volume_volume_lon1_01
mount -o discard,defaults,noatime /dev/disk/by-id/scsi-0DO_Volume_volume-lon1-01 /mnt/volume_lon1_01
Copy data from current storage to the new. I should say that I did this when no services were running on the machine. If you do this to a fully operational system, you should make sure that all services that work with the data are temporarily stopped.
cp -av * .* /mnt/volume_lon1_01
Fix the top-level permissions of the mounted volume.
chown user-data.user-data /mnt/volume_lon1_01
Now move the old user-data out of the way and mount the new one in its place:
mv user-data user-data.old
mount -o discard,defaults,noatime /dev/disk/by-id/scsi-0DO_Volume_volume-lon1-01 user-data
Add a line to /etc/fstab to auto-mount the volume on reboot:
echo '/dev/disk/by-id/scsi-0DO_Volume_volume-lon1-01 /mnt/volume_lon1_01 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab
RE-add deleted UFW rules
ufw allow in 53
ufw allow in 25/tcp
ufw allow in 587/tcp
ufw allow in 993/tcp
ufw allow in 995/tcp
ufw allow in 4190/tcp
ufw allow in 80/tcp
ufw allow in 443/tcp
ufw limit in 22/tcp
*** Obviously your volume names etc may differ.