This is at the 'thought experiment' stage, and I'm not running a serious experiment.

The genesis of this is that I want to have some data available on both a RazPi, and a linux box, and I wondered if it could be done with one live usb key.

The RazPi boots without grub, or sector 0, AFAICT. The 1st partition has to be FAT32. There's a string of gpu firmware, memory allocation, cpu firmware, memory reallocation, peripheral firmware loaded & dealt with; there's several programs on /boot, one f which is loaded that does stuff, and a config.txt that's the equivalent of the BIOS peripheral setup. That config.txt loads the kernel which has it's name mentioned in config.txt. There is a file 'cmdline.txt' which is passed/appended to the kernel providing it's boot parameters, including root=/dev/somewhere. A System.map has to exist for this kernel and is loaded. The entire package in /boot is about 40Megs, and you'd need a little headroom - say 50Megs total. This cannot have grub or lilo (which each think they are the only one about) asking them questions in X86 code.

Linux needs grub, lilo, or something. I'm no longer an expert on non-conformist booting. Alien's live usb, for instance, has a 1M boot partition that's teetotally blank . I've been sticking to old style techniques, and they have served me well.

There's very little room for maneuver in RazPi booting; I can specify a root=/dev/somewhere, but the rest has to be as I said.

So could I have the following setup?

1. RazPi boot
2. x86_64 boot (perhaps with EFI). I have it disabled ATM.
3. RazPi /
4. X86_64 /
5. DATA

The RPi boots by reading the FAT partition & then transfers to the second partition, Linux can boot any partition, & as far as I know, if you add a boot loader for it in the MBR, it should be ignored by the RPi. So, I think you could reduce the size of the RPi / partition, & then install Linux normally, pointing the Linux install to your /home - however, there will be dot files in your /home for both RaspiOS & your Linux version, they may clash.

Probably better to allow both to have their own /home, & have a separate partition for shared data.

1 members found this post helpful.

The big issue is That RazPi needs to ignore a grub MBR. I have posted this query on the slackware arm forum

So, having had my interest piqued, I gave it a go....

Yes it works!

First, dd a RaspiOS image to the drive, before restarting, go to /usr/lib/raspi-config/init_resize & change the line starting with
to something that will prevent it using up all the disk, these are in 'sectors', (& I think RaspiOS is using 4096 as the size).
I just stuck in TARGET_END=$((8000)), which prevented it from grabbing all the disk.

I then ran RaspiOS to check it was working OK, it was using about 3.3GB.

Shutdown RaspiOS, then install Linux to the rest of the disk as normal, I used AntiX, (my regular distro), & let it install grub to the MBR.

The RPi that I was using just booted up normally from my USB pendrive, & when I inserted into a regular computer, grub booted up Linux, no problem.

Edit: I was using RaspiOS64 on my RPi3B+

2 members found this post helpful.

Great!
/moan
I'm an stroke victim here, so me trying things can be a big deal, largely because your work rate is _so_slow.
/End moan.

It's good to know the idea works. Thanks for trying it.

Your welcome, it was an interesting problem that needed answering.

I found it interesting also.

Dooh!! wrong thread.