My current HC4-setup consists of 1 SSD and 1 HDD. The SSD contains the most recent Armbian-image (Buster-based) (https://redirect.armbian.com/odroidhc4/Buster_current).
Long story short: it won't boot from SSD, and Google tells me it is because Armbian is not compatible with Petiboot. Armbian suggests the following solution:
"Important: To be able to boot clean Armbian mainline based u-boot / kernel experiences, you need to remove incompatible Petitboot loader that is shipped with the board. From the Petitboot, go for “Exit to shell” and these commands to remove the Petitboot: