"Apple's APFS is still a work in process! Fusion Drives & RAID are still not supported yet.
In fact if you have a SATA SSD you're better off staying with GUID & Mac OS Extended (Journaled) for now as APFS has issues with queuing across a SATA interface (not an issue with PCIe SSD's).
You encounter time-points when you are saving a snippet, URL link or a file and the system will pause the action, yet the process you are running won't have any problems. I've see it take almost 30 sec for the file to show up on the desktop! Yet, if you do the same thing running the same actions on a PCIe SSD it won't stall-out."
1. Clean Install: Install to a standalone drive (separate from your SSDs), then use Disk Util to create or format your two SSDs RAID 0 and make sure it is formatted Journaled HFS+. Then clone the single drive to the RAID 0 volume and it will work.
I had tried this method using APFS on the RAID and it fails to boot. So the issue is APFS *can't* boot from RAID.
2. You can try upgrading to High Sierra, but without converting the file system to APFS. (I have not tried this myself).
Download High Sierra, but do not install it, then from the command line run:
/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/startosinstall --converttoapfs NO --volume /DestinationDriveName"
Just to tell you that the clean installation and update on a 21.5” iMac Mid 2010 (11.2 ) with x2 SSD Kingston SSD’s in RAID 0 16 GB RAM and has worked perfectly with the MacOS High Sierra Patcher Tool for Unsupported Macs, using HFS+ (not APFS)
We have tried it as a resource since High Sierra does not allow to install APFS on RAID volumes. Just Use: http://dosdude1.com/highsierra/
"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:
When you create your RAID volume, make sure it is in Mac OS Extended and not APFS.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."
