While upgrading my Mac Mini Server/RAID-0, I encountered the High Sierra installer message: "You may not install to this volume because it is part of a AppleRAID.". As well, a OWC Guardian Maximus/2x 4TB/RAID-1 is attached providing a bootable backup for the RAID-0. High Sierra is successfully running on RAID-0, my process:
Notes:
1. RAID-0 is the Mac Mini/serverOS
2. RAID-1 is the Maximus/serverOS-copy (daily backup)
3. CCC - Carbon Copy Cloner
Steps:
1. Disk Utility: Erase RAID-1 (daily backup)
2. CCC: RAID-0 > RAID-1
3. Disk Utility: RAID-1/+serverOS2 partition
4. CCC: RAID-0 > RAID-1/serverOS2 (second backup, if serverOS/10.13 is not bootable)
5. High Sierra Installer: RAID-1/serverOS-copy/10.13 (verify bootable)
6. Disk Utility: Erase RAID-0/serverOS/10.12.6
7. CCC: RAID-5 > RAID-0/serverOS/10.13
8. Boot: RAID-0/serverOS/10.13 (success)
Summary:
1. 10.13 installer will not install to Apple Software RAID, purposely?
2. 10.13 runs on HFS+/Apple Software RAID (install to non RAID, copy to RAID)
3. No 10.13 3rd party hardware RAID issues
4. Second backup is not necessary (Steps 3 and 4)
5. CCC/erase before copy:
a. 10.12.6 serverOS: 88 GB
b. 10.12.6 serverOS-copy: 167 GB
c. 10.13 serverOS: 22 GB