TL;DR: Do. Not. Share. Boot. Partitions. Across. Linux. Installs.
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So a couple days ago I posted a screenshot of my first Gentoo install in a VM. I mentioned that I planned on putting it on my hardware in a triple boot setup (Arch - Main, BillyG-O$ - VR games, Gentoo - Pain). This is a follow-up on my adventures doing that.
Everything went well initially, I followed almost the exact same steps as the VM and things went by much faster (as it had access to all my resources). However I made 1 respectfully sized blunder, for some stupid reason, I tried to share boot partitions between my Arch and this Gentoo. DO NOT DO THAT, I can speak from ~this~ experience.
Because of sharing boot partitions, GRUB got absolutely buggered 6 ways from Sunday resulting in GRUB unable to find my Arch initramfs. Not good at all. So for about 2 days my system was unbootable (I took a break from the computer, I'd just about had it).
Thanfully though it turns out my previous Arch-only GRUB install still existed so with a quick boot override I was finally able to get back into Arch. I then proceeded to totally destroy every last trace of Gentoo from my boot drive and reinstall the kernel, thereby remaking my initramfs. My system is restored 🥹.
So, in conclusion, I feel that Gentoo is definitely good fun and worth doing, however if you're installing it on the same drive as a pre-existing OS be. Hyper. Vigilant. I'm not sure how or why my old Arch-GRUB still existed but by golly am I thankful it did.
This did not scare me off Gentoo at all because it was my error. So I'm probably going to reattempt it soon. I also jusy wanted to say thanks for the engagement and tips on my previous post, I love the Linux community and our little corner of the internet. I await the day where our pride and joy is mainstream, along with so many others I'm sure. I hope you have a great day/night :).
I've definitely made similar errors dual booting. I don't know what your budget or hardware is like but if you can fit a second hard drive in your computer it is very worthwhile for distro hopping.
The second hard drive will let you install whatever boot loader the USB installer wants to install without overwriting the bootloader on the first hard drive.
Modern grub is pretty magical and should detect both OSs either way.
But yeah that's why there are so many warnings in every Linux distro about installing grub. You do not want to mess with a currently working bootloader.
This is mostly because the bootloader needs to be in a very specific location on the hard drive and so you can't just install it anywhere. Which means you have to overwrite the old boot loader if you choose to install a new one.