Same issue happened 1-2 years ago. I have triple boot system with windows, arch and nixos. I install nixos and use it no problem for few days or a week, I switch between all systems daily but then suddenly nixos partition goes blank, no info about it, bootloader cant load it, cant mount it. What I did before this happened I deleted bunch of files in my windows installation and then did optimize/defrag C disk.
Nixos partition /dev/sdb6 30gb size
I tried to analyze with testdisk tool but it couldnt find anything. I tried creating linux ext4 partition defined sectors of it but still nothing works.
Arch and windows still work just fine. I have been working on my dotfiles for days and didnt put them into git repo yet since I wanted to have system with all apps I need before doing backups. Atleast I would like to access and save my configuration files.
Do I lose my nixos yet again? Why this happens? I didnt touch/resize any partitions since nixos installation