Hi, just writing it here in case you were curious of the cause of the problem.

I finally had some time today to work on it. What I tried was to copy the content of the system boot partition of another PI, one that I configured at the same time/day, and compared file by file with the content of the partition in the broken one. Now there were some diff in the some binary files as expected, but what surprised me was that one file was ONLY present in the working one… initrd.img XD Now, don’t ask me why the hell the file was not there. Maybe it got corrupted somehow, since nothing touches this partition as far as I am aware aside at boot time. Luckily, there was .bak file present in the partition which I renamed and… it worked.

Lesson learned: I now have a copy of the boot partition of every PI I managed (it is only 167MB pre tar-zip, so it is not a cost) and I have it backed up safely on another system. Should another file get corrupted in the future (maybe this time without a .bak) I have an older working copy if it, and I can restore the service without the need to format everything.

Thank you so much for your help and advice!

I could format it and try but the moment I do I lose the ability to debug the issue and learning from the problem. So I may temporary solve it but it may happen again. I wonder if someone knows what I can check/test/run to identify what broke.

Thanks for the quick reply 👍

The next step I would try would be to boot an other install, like a liveusb or a raspbian, on the same usb port, to completely eliminate a hardware problem if it boots properly.

Good advice. I moved the usb drive from another (working) PI and attached it to the same USB port. It boots correctly. It is not the USB port nor power.

If it is a software problem, it seems to happen very early in the boot process, so my bet would be a corrupted initramfs/initrd (or what is equivalent on a Pi). No idea how you could debug and fix that on Ubuntu, though (especially on a Pi where /boot is… different).

I believe it is something like that. Or it is not mounting the drive correctly and not finding it, or it is something else. I just wish there was a better (or any) error printed on the console. I tried to attach a keyboard to get to a shell with no success. I honestly could just reformat the drive and use a clean install, but it is the last resource. I would like to understand what happened so I can learn from it and avoid it in the future (or learn a path to fix it).