Grub error after installing Qubes 4.2 on external SSD

[apparatus]

The issues that I linked are only applicable for the EFI systems so that problem is not the issue that you’re facing.

You can try to fix up your like this:
First boot into any LiveUSB OS or in Qubes OS installer, mount your system (either manually or automatically if you’re booting from Qubes OS installer in rescue mode), chroot into it and then fix grub config:

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…