Stuck in Grub Recovery after installing Second OS to alternate Hard drive. How to recover?

[apparatus]

Maybe your second OS overwritten your Qubes OS grub config.
I don’t have PC with legacy BIOS to test it but you can try to restore your Qubes OS grub config by decrypting Qubes OS dom0 root, mounting, chrooting into it and then running grub2-mkconfig there.
You can mount and chroot into dom0 root like this (this guide is for EFI so you’ll have to adapt it for legacy BIOS):

Reversing qvm.usb-keyboard

You can install the grub again. Boot from some LiveUSB (e.g. from Qubes OS installer in Rescue) and chroot your Qubes, assuming /dev/sdb is your Qubes drive: Read how to mount your root: How to mount a Qubes partition from another OS | Qubes OS Mount your system: mkdir /mnt/system mount /dev/qubes_dom0/root /mnt/system cd /mnt/system mount /dev/sdb2 boot mount /dev/sdb1 boot/efi mount -t proc /proc proc mount -t sysfs /sys sys mount --rbind /dev dev mount --rbind /run run mount --rbind /sys…

Then try to fix your grub like this:

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…