I had a thought! If you have a full backup lets give this a try:
High Sierra offers a new file system APFS which is better for SSD's. It also could be the answer here. But, to get there we'll need to break the drive set so you can reformat the drives with APFS. Once you get both drives redone using an external boot drive go into Core Storage command line to recreate the RAID set. Then do an OS install. That should do it.
+
+
Just remember! As @mayer stated this shouldn't be your primary system. As if this fails you'll need to roll things back!
I had a thought! If you have a full backup lets give this a try:
High Sierra offers a new file system APFS which is better for SSD's. It also could be the answer here. But, to get there we'll need to break the drive set so you can reformat the drives with APFS. Once you get both drives redone using an external boot drive go into Core Storage command line to recreate the RAID set. Then do an OS install. That should do it.