I did something similar a couple of years ago when I installed Arch on a
PC with Debian-Buster and a BIOS Legacy option.
I can't say whether this procedure will work with UEFI, since I haven't
tried that. You proceed at your own risk.
1. Power the computer OFF and disconnect all external cables.
2. Remove the cover and disconnect all hard drives from the motherboard
(i.e unplug the SATA cables)
3. Connect the target hard drive to the first SATA port on the motherboard.
4. Reconnect only the essential external cables (power, keyboard,
mouse, monitor).
You can leave the cover off for the installation bit.
5. Insert ISO-stick or ISO-DVD, power-ON, and install in the usual way
on what is now the only hard disk
6. When the installation is complete, reboot the PC to check the
installation works.
7. Power the PC OFF, remove all external cables as a safety precaution.
8. Reconnect all hard disks in the preferred order, and put the cover
back on the box.
9. Connect all external cables and start the PC.
10. Run update-grub (as sudo/root). If all goes well Grub should run
os-prober and find both operating systems.
It worked for me, but your mileage may differ
Good luck
Dynosaw
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