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I have an OpenHab 3.1 installation on a raspberry 4 running. I'm currently using raspiBackup to create a full backup of the system and also having an .img file to restore it when needed via Etcher under Windows.

Now I wanted to give the restore a try and boot it from a raspberry 3b. But all I get is a black screen. I checked the /boot directory and it seems bootcode.bin and start.elf are missing on the origin system. Adding them manually from here to the restored boot partition shows still a black screen, but at least the green ACT led is blinking 4 times - which basically means it has issues loading the start.elf - as far as I understood it.

My actual question is now, is it even possible to run a RasbianOS installed on a rpi4 on a rpi3b? I'm booting from a SSD directly connected via the same SATA2USB adapter - in both cases. If I do a fresh RasbianOS install using the rpi3b, it boots properly.

javg
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    This Question is hopelessly confused and what you appear to be asking has nothing to do with the title. – Milliways Sep 09 '21 at 00:09
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    Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Sep 09 '21 at 20:31

2 Answers2

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You can run Raspberry Pi OS (32bit) on any model Pi.

Whether you can boot from a SSD depends.

Milliways
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  • The actual question is, if an installed pi4 system can simply boot by replacing the pi4 with a pi3b. For me this doesn't seem to work. – javg Sep 09 '21 at 20:16
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    I understand that the last part (boot from an SSD) is exactly the case the OP is having. It would be good to know on what it depends. – Dmitry Grigoryev Oct 13 '21 at 10:21
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Although there was not much help coming from here what to check/try to come closer to the root cause (especially for the black screen), I want to give my findings back so someone else might find it useful.

To make it short: it is possible.

I had to use serial debugging though to find out, why there wasn't even the rainbow block showing up - but just a black screen. It gave the necessary hints why it wasn't booting.

First, my rpi4 image was trimmed down during installation of Openhab to just support the rpi4 as it seems. So e.g. required files for the rpi3b like bootcode.bin, start.elf and kernel.img/kernel7.img were missing on /boot. But just copying them over from here didn't work. Serial debugging showed, it was failing on .dtb files and after replacing them also overlay files. So I just replaced all files in /boot with the raspberry Github repo boot in the end.

Another issue was, that I had set gpu_mem=16 in the rpi4 image config.txt that the rpi3b didn't like. Sadly there was no indication - even with serial debugging about this. It was just stopping after initializing the uart stuff. Take-away from this is, doublecheck your cmdline.txt and config.txt for maybe "harmful" settings that might not be supported by the rpi model you're trying to boot from. Try to comment them out and see if it boots further or not.

It can also be helpful to understand the boot process in general and what are the files in /boot used for.

After adapting those things, the rpi4 image was booting properly on a rpi3b (with USB boot already enabled).

javg
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