I have a Home Assistant instance running on Raspberry Pi 4 with HaOS, booting from an external 500GB SSD. Everything worked fine until it died 
The system is running in a location cca 200 km away, and I was not there to troubleshoot it on the spot, so I had the device brought to me.
The system is not booting at all, the HDMI output just blinks and that is all. If I disconnect the drive, it goes to RPi “boot” screen saying that it can not find a drive to boot from. As soon as I connect the drive it says that it found config.txt, etc… and it seems that it starts to load u-boot or something but the HDMI output goes away and never comes back. I tried connecting UART to see if it outputs anything but it does not.
I should mention that the disk works fine if connected to my Linux machine, and I was able to make a complete image of if. The system also boots Raspberry Pi OS from another USB flash drive. I also flashed a new HaOS image using Balena etcher, and it seems to be working (restoring from backup as I write this).
The boot process looked very similar to the failed boot of the SSD. I disconnected all USB devices, waited for the RPi boot screen and then I connected the flash disk. It found the config.txt, etc. and the HDMI output went black but after a few seconds it came back and I saw it booting…
So basically, I would like to know how to debug the boot. I suppose I can just flash a new image to the drive and restore from backup, but I would like to understand what went on, because I would like to avoid the problem in the future.
Also is it normal that the drive has 8 partitions on it? While cca 6 of them are few MBs squashfs?