Dual boot a RaspberryPi
 

So, having had my interest piqued, in another forum thread 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+

<< Posted in the hope that it may be of interest to others. >>

You could also manually fdisk/cfdisk and resize2fs the / partition to accomplish the "extra" space/partition on a single storage device. Linux is all files on a filesystem so you don't even need to be using an arm computer to do that little trick.

I've also cloned the install to another SD card, then stripped everything away except /boot and the /lib/modules/ for the raspbian kernel, then installed debian via debootstrap. That was debian armel (armv4) long ago before raspberry pis had good GPU support for 3D things. So that for the software (mesa) drivers of 3D. For 11fps in glxgears, eventually 17fps in raspbian when GPU support allowed it. The original B version with a single 700MHz armv6 CPU.