add howto to repair nixos installations

Author: Mic92

docs/howtos/INDEX.md

@@ -12,6 +12,8 @@
 
 [Using your own kexec image](./custom-kexec.md)
 
+[Repair installations without wiping data](./disko-modes.md)
+
 [Secrets and full disk encryption](./secrets.md)
 
 [Use without flakes](./use-without-flakes.md)

docs/howtos/disko-modes.md

@@ -0,0 +1,16 @@
+# Repair installations without wiping data
+
+By default, nixos-anywhere will reformat all configured disks before running the installation.
+However it is also possible to mount the filesystems of an existing installation and run `nixos-install`.
+This is useful to recover from a mis-configured NixOS installation by first booting into a NixOS installer or recovery system.
+
+To only mount existing filesystems, add `--disko-mode mount` to `nixos-anywhere`:
+
+```
+nix run github:nix-community/nixos-anywhere -- --disko-mode mount --flake <path to configuration>#<configuration name> root@<ip address>
+```
+
+1. This will first boot into a nixos-installer
+2. Mounts disks with disko
+3. Runs nixos-install based on the provided flake
+4. Reboots the machine.