Can’t boot from RAID

This is my first attempt at using RAID. Does anyone know what’s going wrong? I looked online and it seems I should be able to use a “RAID set” like my disk4 as a boot drive, but when I try to boot from it it just tries for a minute then shuts down.

Details:

El Capitan (10.11.6) is the newest OS Apple allows me to use on my ’08 Mac Pro. So I used Terminal to set it up. I have 4x7200RPM HDDs, 1x16TB drive intending to be partitioned into 13TB to be used with Time Machine, and 3TB to be used with the other 3x3TB drives for RAID.

First I partitioned the 16TB into 8TB disk3s2, 5TB disk3s3, and the 3TB disk3s5 for the RAID. I copied my old solid state boot drive to disk3s3, and all the data on my other drives (previously just used as storage for games and personal files) I organized into that new boot drive. I also have over 3TB of games uninstalled right now that I’ll reinstall when all this is done.

Then booting from the new boot drive I erased and reformatted my old one to use in my laptop, shut down, and replaced it. So I just have disk3 and the 3x3TB empty drives for the RAID in. Then back on disk3s3 in Terminal I did:

diskutil appleRAID create stripe Celarian JHFS+ disk0 disk1 disk2 disk3s5

This seems to have worked. It created a new disk4 named Celarian with 0 used of 12TB space. The solo partition disappeared from below each of the 3x3TB drives, and disk3s5 also disappeared from below disk3. However under External there is an Untitled drive with a different icon from the internal ones that when selected loads nothing and has 4 partitions. They do load and show as disk0s2, disk1s2, disk2s2, and disk3s5. All of which are named Untitled as well, and show as 3TB of 3TB used, as does the 3x3TB internal drives.

I booted into Recovery Mode, opened Disk Utility, and did Restore on disk4 copying from disk3s3. This kind of worked… I can select disk4 and see it has the right amount of data used on it, but it has no partitions. Loading into disk3s3 I can see Celarian (disk4) as an option in Volumes, and it has a perfect copy of disk3s3 on it. It even appears in System Preferences under Startup Drive options.

However switching the boot drive and restarting lead to my computer being unable to boot up at all, even into Recovery or Safe Mode. The way I finally got it to work is by booting it after I pulled out all 3x3TB drives (removing 1 didn’t work). So that it would default back to disk3s3.

I checked Disk Utility, but the First Aid option (and Partition) is greyed out on disk4; even if I try from Recovery Mode. The only thing left that I could think of is trying to Restore disk4 from disk3s3 again. That’ll be done in yet another 24h I assume. Help and advice from anyone familiar with this situation would be greatly appreciated.