Thanks for reporting this @Patrick_Martin. As you’d understand, it’s hard for us to ensure hardware compatibility as we all run Rockstor on a large variety of hardware. You can ignore the warning. If you want to remove the warning and make an older kernel default, see this post.
I hope that with 4.4, we could stick to an LTS kernel and not have to change often.
Slightly OT, but I encourage those of you that are getting a bit deeper into BTRFS to report problems on that project’s mailing list, which could help out BTRFS developers.