I was able to upgrade my Apple RAID 0 set (SSD) in my 2011 iMac from OS X Sierra to OS X High Sierra by the following: I downloaded (didn’t install) OS X High Sierra. High Sierra won’t directly upgrade an Apple RAID 0 volume. Cloned (Carbon Copy Cloner) my OS X Sierra RAID 0 set to an external (thunderbolt single drive). Opened and started High Sierra installer and upgraded the external (cloned) drive to High Sierra. Reboot into the High Sierra external drive. Erased RAID 0 set using disk utility (High Sierra). Cloned the external drive (High Sierra) onto my internal Apple RAID 0 set. Disconnected external drive and rebooted my iMac into High Sierra system. Success (so far). All programs and files are functional so far. File system isn't changed to APFS Note: I have redundant backups and clones just in case this didn’t work! So far, success.