1. I went to settings -> docker and set enable docker to "no". that caused processes hanging on to /mnt/disk*/system/* to exit - probably the loopback mount of docker.img onto /var/lib/docker.
2. Then ran mover and that copied files into docker.img and libvirt.img into /mnt/cache.
3. Then set enable docker to "yes" and apply.
Then spin down disks and confirm they don't spin up again. You can verify what files are hitting the disk with 'lsof | grep /mnt/disk'.
In my case i still had a grafana/influxdb container accessing files in /mnt/disk*/appdata because i forgot to re-set "cache: prefer" for that, so I had to take a further action of stopping that container, running move again until it was complete and starting container back up. A subsequent 'lsof' confirmed no /mnt/disk references and exclusively /mnt/cache/appdata instead.