Hi raulfg3. Thanks for replying.
In the end, I solved it with a combination of measures. Every time I formatted a disk as ZFS Pool Storage, it would be "freed" and taken by the RAID5. If I at this point did this:
I would see that there was still GEOM/UFS/RAID5 metadata on the disk. So I speculated that this would remove the metadata:
and running dump
again verified that it did. I repeated the cycle for all disks (format, remove, dump), and in the end I removed the whole RAID5 using
and this time it STAYED down.
However, isn't this a bug -- or maybe lack of required feature -- in NAS4Free?
You must be able to repurpose disks from one use to the other. If you can never repurpose disks from RAID5 to ZFS (or perhaps vice versa or some other combination I haven't tried) without resorting to wiping or having the necessary shell skills, then there's a big-ish hole in NAS4Free's functionality.