Installing NixOS on two different partitions

I currently have NixOS installed along with Windows and want to add another NixOS installation on a different partition because the current partition has grown too small and I am not ready to switch at once. I have therefore installed NixOS on another partition as described here.

My issue is that I am completely unable to boot into the new system. I am using grub with os-prober, which seems to find the new configuration (Found NixOS 23.11 (Tapir) (23.11) on /dev/nvme0n1p7) but not add it to the boot menu like with the Windows installation. The additional entries in boot.loader.grub.extraConfig either jump back into the same boot menu, panic because some files can’t be found or boot into the old configuration.

In case it helps, this is my config.

Any help is appreciated!

not add it to the boot menu like with the Windows installation

Are you on uefi? I think os-prober can only find windows (or lilo) with efi boot.

I am not using legacy boot if I understand your answer correctly.

test -d /sys/firmware/efi && echo "UEFI" || echo "Legacy"

But it does find the system so it should add it, right?