Cannot boot R4.1 after installing R4.2 on external ssd

[apparatus]

You can try to fix boot entry like this:

BIOS Update That Will Break Boot

If you use uefi : get a qubes os media installation. boot to qubes os rescue mode. skip to shell. [shell] : efibootmgr -v -c -u -L "Qubes OS" -l /EFI/qubes/grubx64.efi -d /dev/nvme0n1 -p 1 reboot change -d <which drive?> and -p <which efi partition?>. if you want you can make sure first by issuing efibootmgr -v in dom0