20.04 boots from SSD but not if HDDs are online
You probably have the BOOT flag set on one or more of the partitions on the 8x4TB drives.
Unmount (umount) them, and use gparted to clear the BOOT flag(s) on the 4TB drives.
Your BIOS steps through the disk controllers, steps through each disk connected to the controller to find the fIrst partition with the BOOT flag set, and boots from that partition. If there is a system there, all is good. If there isn't, all is not good.