Qubes grub installation failed [dual booting w/ arch] and fails to boot when fixed [/system doesn't look like OSTREE]

[apparatus]

@albassorty

(if you’re using UEFI)

No, GPT

I meant did you boot in UEFI or Legacy BIOS mode, not your disk partitioning scheme.
Do you have EFI System partition on your disk?

I guess you’re using legacy boot mode. Then check this guide:

Multibooting Qubes

Introduction You should think carefully before dual booting Qubes on your box. Read the guidelines carefully. One problem is that when you dual or multiboot, even if you are using encryption on your Qubes installation, /boot is still unprotected and could be maliciously modified by the other OS, possibly leading to Qubes itself being maliciously modified. The other problem is firmware security - for example the other system could infect the BIOS firmware, which might enable compromise or spyi…