You can change between any RAID level as far as I know. Start with your one disk and put some data on it. When you add another disk (or disks) in the future, you can add them to the “pool” that your original disk is in and then change the pool to a more stable RAID level (like 1, 5, or 6) once you have enough disks to provide redundancy.
I realize that this might sound confusing when you read it, but when you see the Rockstor User Interface it will be SUPER simple to do what you want.
I forgot to mention that if you change your mind after you’ve converted to Raid 5 and decide you want to switch back to RAID 0 or upgrade to RAID 6, you can always switch again. No limits. You’re not locked in like with ZFS or mdadm.
Please note that changing RAID levels needs some time to rebalance, i.e. create parity, duplicate data or striping the data depending on the level your aiming for. This might take several hours, depending on the amount of data and the speed of your disks (and not on the size of your disks as with other raid solutions). If I remember correctly rockstor’s UI doesn’t correctly reflect the balance-status atm.
So I wouldn’t go around and change the raid level several times an hour, nobody knows what will happen.
@Arneko is correct. You do have to give the system time to adjust to your new settings. Don’t click between RAID levels for fun because you could run into an issue if it is not done converting when you change again. Assuming that you need to do something different a few weeks down the line, though, Rockstor will allow you to do that without restriction.
Note that RAID 5 and 6 are still considered experimental in BTRFS. Many people are using it successfully, but it has not been perfected at this time. I am using RAID 1 because it is complete, and will convert/expand to RAID 6 when the BTRFS devs give the thumbs up. I have data that I can’t afford to lose on my Rockstor NAS, so I’m being very gentle with my testing.
This is not a Rockstor limitation, it is a BTRFS limitation. I am expecting that RAID 5/6 will be 100% ready in the next few months, so it could be stable by the time you are ready to add your extra disks.