@Tex1954 Hello there.
I don’t think I quite get your question/difficulty here. Sorry. But any pool in Rockstor can, as per btrfs, have any size drive added to it. Can you explain more what restriction you have running into.
Btrfs and Rockstor in this case (we don’t represent all of btrfs but drive adding we do) has no real ristriction on adding a drive to a pool. What I think may be happening here is a difference in interpretation re raid in old style hardware and raid in software and specifically btrfs.
Btrfs raid 1 for example can have not only 2 but any number of disks. But the ‘software’, read btrfs drivers, ensure that there are exactly 2 copies of each data and metadata on 2 different devices; indipendant of the number of devices. There are now 3 and 4 copy variants but we don’t yet ‘understand’ them in rockstor.
So put more simply raid in btrfs is at the chunk level, not the drive level. This enables things like different raid levels for data than for metadata i.e. raid1 using dup metadata for example while using ‘raid1’ for data.
So that may all be irrelevant here but I thought this may be a potential sticking point. I.e. data is mirrored at the chunk level not the drive level, at least in btrfs raid1/10 for the data. And raid levels are set at the pool level and new files will conform to this level, at least until a balance is performed that, depending on options, will re-write old data as new and thus move it to the new raid level. Rockstor does an auto ‘whole’ balance post a disk add so that is already taken care of via the Web-UI.
Hope that helps. And to change / add a disk you go to the pool details page and there’s a button at the bottom which in turn presents a set of Pool modification options, add remove change raid level.