Yesterday i was forced to hard-reset a frozen
#FreeNAS box. After the reboot the
#Nextcloud dataset was empty.
Free space on disk was still the same, and a
#zfs scrub found no error, so where had the 600GB gone...?
It turned out the NC dataset had been created, but not properly mounted. Instead the NC data ended up in a simple folder on the root dataset. After the reboot the (still empty) Nextcloud dataset was auto-mounted over that folder, so all the files were hidden.
#ZFS never forgets :D