This situation is a BIOS related issue. The macOS update may change the BIOS code in iMac. Please login the web manager console of the raid box and modify the BIOS selection setting to disable to see if it makes difference. You can find the BIOS setting feature in Advanced Feature menu.
If you use ArcSAP for management, please choose Overview tab, Advance Feature to be able to open the web manager console of this raid box.Tags: ArcSAP, BIOS, iMac, macOS, Thunderbolt3