

Remember, once you install High Sierra on the target drive, you’ll be able to access your files in the RAID set again. Unless you have enough room to transfer your entire system, resist the temptation to pull over tons of files when the “Transfer Data to your Mac” assistant pops up during installation. You can even use an external drive, but make it a fast one, because it’s going to be your new boot drive (alternately, if you have a really big extra drive, you can clone your data to that drive, install High Sierra on that, then clone everything back to your RAID set).

The installer needs 5GB of space, so plan accordingly. If you're lucky enough to have additional storage installed on your iMac, you can load High Sierra on that drive.
