The external drive enclosure has been the problem for me before, if you've ruled out everything else. It was only a problem on the Pi, and only a problem when trying to boot from it. I've also had it not boot unless it was on a powered USB hub, so that's worth a shot as well.
Discussion
This is helpful (and curious), thanks. The drive has worked without problem when it was just mass storage for raspitblitz.
Any progress?
Does the pi have different kinds of USB ports or are they identical in all ways?
Solved. 👍🏼 Pi 4 has 2 USB 2 + USB 3 ports.
@note1fkdrldx478xfvsxsmja2y9q7hk9u4jzzd4wg4h4q4hwmsgwxedast0sdar
Wow! What the "f" is that all about?! :) Congratulations!