Dual boot Nixos with Arch Linux, Arch isn't in grub menu

I’ve been trying to install a dual boot system with nixos and Arch. I’ve successfully installed both (Arch and then Nixos) but my current grub menu only shows Nixos. Here’s what I did.

Set up partition table:
1 MiB unallocated space (gparted wouldn’t let me use this first 1 MiB.
sda1 2MiB, no file system, no mount point, BIOS_grub flag
sda2 512MiB, ext4, mount point = /boot
sda3 75GiB, ext4, for nixos
sda4 75GiB, ext4, for arch
sda5 2.7TiB, ext4, mount point = /storage
sda6 8GiB, swap

I SUCCESSFULLY installed Arch on sda4 and it created a grub menu that appeared on boot with the Arch installation.

I SUCCESFULLY installed Nixos on sda3. During the installation, I did NOT mount sda2 or sda4. When I rebooted, I got a grub menu, but it did NOT have the Arch installation.

I SUCCESSFULLY RE-installed Nixos, again on sda3, but this time I made directories /mnt/boot, /mnt/arch and /mnt/storage. I mounted sda2 at /mnt/boot, sda4 at /arch and sda5 at /storage and completed the installation with no errors. Again, there was no Arch installation in the grub menu. As expected, /arch and /storage were both present in the file system when I looked using the file manager.

How can I get Arch to appear in the grub menu?

As you are using grub you can set boot.loader.grub.useOSProber to true. https://nixos.org/nixos/options.html#boot.loader.grub.useosprober

I forgot to include in my original post that I did include that line in my configuration file and I also added os-prober to the packages to be installed. Though I did this during the installation, Arch didn’t appear in the grub menu.