If /srv/empty existed and was an empty directory, then snapraid sync -E should have worked and removing the disk 4tbdata01 entry in snapraid.conf should have been the end of it.
Yes. I'm puzzled. The chances are that i've been fat fingered or distracted.
I think I could probarbly re-add the drive and try the procedure again, but having gotten this far, is there another way of telling OMV that the drive is no longer part of Snapraid which would allow me to run the maintenance commands on the rest of the array?