LOL yep that snuck through, it will be corrected in next release. Explanation: this release -rc13 had all the code in place to support "cache pool", that is, you can assign a storage device (HDD or SDD) to either the "array" or to the "pool". In addition you could configure the "pool" to use btrfs as the file system. So if you want, you could assign all disks to array, or all disks to pool or some combination of both.
Unfortunately, this resulted in quite a few changes to the webGui which broke a number of plugins, particularly Simple Features. So I've ripped all that out until I can coordinate with plugin authors. This was an "executive decision" that delayed the release by at least a week.
You aren't going to remove the 6th data drive are you? I hope not, I've just spent the last 2 hours putting a new drive into my server and neatly arranging all of the cabling.