Use SuperDuper! to clone a disk with High Sierra to the RAID volume. You cannot install High Sierra directly to a RAID volume. However, if you use SuperDuper! (which is free for this, btw) you can simply copy/clone the drive with the working High Sierra over to your RAID volume. Some important points: (1) When you create your RAID volume, make sure it is in Mac OS Extended and not APFS. (2) You can actually be booted up in the High Sierra install that you’re intending to copy _from_ when you do this. I’ve done this and it works great. Mojave might allow you to install directly to a RAID volume but I couldn’t do that because I was waiting for Nvidia to release their macOS video drivers with support for Mojave.