Running on: iMac 27" 5k late 2015 - 64gb ram and a 16tb Pegasus Promise2 R4 raid5 via Thunderbolt. After trying Big Sur - found issues with Luminar Photo app, decided to return to Catalina on the iMac. Reformatted my internal drive and reinstalled Catalina 15.5 and reformatted the raid. But I keep getting the following message upon restarting: "Incompatible Disk. This disk uses features that are not supported on this version of MacOS" and my Pegasus2 R4 portion no longer appears on the desktop or in Disk Utility... Looked into this and discovered that it may be an issue of Mac OS Extended vs APFS The iMac was formatted to APFS prior to installing OS11 so I reformatted to APFS when returning to Catalina. The issues persisted so I re-reformatted from a bootable USB - this time to Mac OS Extended (journaled) and the issues seems to be resolved. The iMac runs slower on MacOS Ext, but it is running and the Raid is recognised... I'd love to go back to APFS but am afraid it will "break" things. Any thought on this would be welcome. Thanks Nick