NixOs does not recognize Archlinux partition

Hello greetings, this is my first post, I installed Nixos on my laptop, there I have Archlinux and Windows installed. When booting it recognizes Windows, but it does not recognize the Archlinux partition, I have not been able to find a way to add Archlinux to the Nixos grub, I use Bios for the installation, I want to get help to solve this situation.
Thank you.

Since Windows was detected I assume you have enabled boot.loader.grub.useOSProber? If not, try that first.

How was Arch installed – did you do anything fancy like LUKS? Since you are not using EFI, NixOS probably overwrote the existing Arch bootloader in the MBR while installing. In that case you will need to manually add an entry in NixOS’ GRUB for Arch using the extraEntries option. It should look similar to the configuration you had for Arch.

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Note that os-prober only detect other linux systems on legacy bios. On UEFI it only detects other windows installs (and lilo, but it’s rarer).

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