Hello guys and Happy New Year!
I've just installed NixOS on PC where I have NVMe SSD With Windows 10 and HDD with NixOS. SSD structure is:
    /dev/nvme0n1p1      2048   1050623   1048576   512M EFI System
    /dev/nvme0n1p2   1050624 975818790 974768167 464.8G Microsoft basic data
    /dev/nvme0n1p3 975820800 976766975    946176   462M Windows recovery environment
and HDD sturcutre is:
    /dev/sda1          34      32767      32734    16M Microsoft reserved
    /dev/sda2       32768 1953529855 1953497088 931.5G Microsoft basic data
    /dev/sda3  1953529856 1954553855    1024000   500M EFI System
    /dev/sda4  1954553856 2373984255  419430400   200G Linux filesystem
    /dev/sda5  2373984256 3907028991 1533044736   731G Linux filesystem
I prefer not to use grub, but default loader do not see Windows partition.

If I try install grub, it says:
    /nix/store/233x9n88dj2kgl9wp08609hz900d368q-grub-2.02/sbin/grub-install: warning: this GPT partition label contains no BIOS Boot Partition; embedding won't be possible.
    /nix/store/233x9n88dj2kgl9wp08609hz900d368q-grub-2.02/sbin/grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
    /nix/store/233x9n88dj2kgl9wp08609hz900d368q-grub-2.02/sbin/grub-install: error: will not proceed with blocklists.
    /nix/store/4jq9lw46w58yd4i9p4kbfh7jfbl8li4y-install-grub.pl: installation of GRUB on /dev/sda failed