Redditor borks his system by completely ignoring the distro's step-by-step installation guide, then copy-pasting terminal commands from random ancient forum posts.
I happened across a mention of a BIOS option that no one else - not the official documentation nor the 99.9% of other forum posters I read - bothered to mention: I needed to switch my hard drive setting from IDE to AHCI.
Because that isn't necessary for the installation and fixes nothing. It adds hard drive hot plug capability if your BIOS supports it.
(By the way, on Windows you get a blue screen if you flip that setting after installation, Linux recognizes it and activates the corresponding driver during boot.)
They switched to AHCI based on a random suggestion they didn't understand, rebooted and found out their HDD mounts are all fucked up.
100% that wasn't the right fix for their boot issue.
Much more likely they just didn't read what the installer asked during the partitioning process. Or did shit they don't mention like move around some HDDs inside the case after installation.
(In fact that's the most likely: unplugged the Windows drive, plugged in a new one, installed Linux on it, then re-plugged the Windows drive and tried to reboot. In that case, you might get the idea that AHCI could help, if you don't know what you're doing. But at that point the boot loader and fstab were already misconfigured cause the fully automated installer couldn't know about the unplugged drive during installation.)