@carl Welcome to the Rockstor community.
I’ve done this ‘partition around duff bits’ myself in the past but that was way before Rockstor and even btrfs itself. Always struck me as a neat trick and wringing the very last use of an otherwise peaky drive (in non critical roles of course) was handy.
Unfortunately, although Rockstor, stable at least, can do ‘btrfs in partition’ it’s really very beta level and just not recommended; the latest stable release of 3.9.2-48 also has a known bug which has been fixed in pending code to be released as 3.9.2-49. I.e. from the pending pr to sort this we have:
Fix prior TODO: re btrfs in partition failure point introduced in git tag 3.9.2-32.
You don’t say which version of Rockstor you are running but this capability should be back in play with 3.9.2-49 at beta level. But it really is best to avoid partitions as it’s just another level of complexity that we just don’t need anymore with the btrfs whole disk capability. But whatever, in stable channel updates version 3.9.2-31 was last known working for this capability.
But in your case, and if you are happy to potentially end up with a broken system, you could try it. See the Disks doc section which has info on how we deal with this. In short it’s essentially via a redirect role: you pick a partition on the disk to use and then use that disk as if it’s a whole disk. Only one btrfs partition per physical device is allowed. Rockstor simply re-directs all request to the ‘named’ / redirected partition. But also very much not recommended really but it did work for quite some time and I intent to give it some more attention once I’m less distracted with current server backend stuff. I’d say post 3.9.2-49 it should be workable again but if you’r game then give it a go. Can’t remember where last testing release was on this front as it’s now rather old but it may have been in there. See Intro section of the following forum thread for our current arrangements re the update channels:
3.9.2-# Stable Channel Changelog
Hope that helps.