Hi! I don't, unfortunately. I'm just learning through trial and error on a VM. I would love to be able to assign Umbrel apps to HDDs. I was thinking I'd just make a RAID and spread it across several, but I'd have to figure that out too.
For that reason, I haven't yet migrated my NextCloud to Umbrel. But I would love to get my NextCloud data out of the cloud to cut costs.