From ffeb6c1007e7364c899bee4128bdc25fd899a3b6 Mon Sep 17 00:00:00 2001
From: Erik Simon <mail@erik-s.dev>
Date: Fri, 24 Jul 2026 21:12:17 +0200
Subject: [PATCH] fix

---
 hosts/terra/disk-config.nix | 33 ++++++++++++++++++++++++++-------
 1 file changed, 26 insertions(+), 7 deletions(-)

diff --git a/hosts/terra/disk-config.nix b/hosts/terra/disk-config.nix
index d222811..949a3e6 100644
--- a/hosts/terra/disk-config.nix
+++ b/hosts/terra/disk-config.nix
@@ -5,13 +5,27 @@
 # `fileSystems.*` entries, so hardware-configuration.nix must NOT define
 # fileSystems for "/" or "/boot".
 #
-# ⚠️  Root is btrfs (was ext4): the disk previously held a CachyOS btrfs root,
-#     and disko's `mkfs` create step SKIPS formatting when `blkid` still
-#     detects a filesystem signature on the freshly-cut partition. A stale
-#     btrfs superblock survives `wipefs`+repartition inside the new root
-#     partition's range, so an ext4 root got skipped and `mount -t ext4`
-#     then failed on the leftover btrfs bytes ("bad superblock"). btrfs's
-#     create runs `mkfs.btrfs -f`, which reformats past any stale signature.
+# ⚠️  disko's `mkfs` create step SKIPS formatting when `blkid` still detects a
+#     filesystem signature on the freshly-cut partition:
+#
+#       if ! (blkid "$device" -o export | grep -q '^TYPE='); then
+#         mkfs.btrfs "$device" -f     # ← -f only runs WHEN this line runs
+#       fi
+#
+#     The disk previously held a CachyOS btrfs root. The whole-disk `wipefs`
+#     disko runs before partitioning clears the signature at the OLD layout's
+#     offsets, but `sgdisk --clear --align-end` then re-cuts the partitions, so
+#     a stale btrfs superblock survives at the NEW root partition's own 64 KiB
+#     offset. `blkid` sees TYPE=btrfs, `mkfs` is skipped entirely, and the
+#     later `mount` fails on the leftover bytes ("wrong fs type / bad
+#     superblock"). Switching ext4→btrfs did NOT fix this: `mkfs.btrfs -f` is
+#     never reached, because the guard is on whether `mkfs` runs at all, not on
+#     its flags. The ESP hits the same trap (its `mkfs.vfat` gets skipped too).
+#
+#     Fix: `preCreateHook = wipefs --all --force "$device"` on each partition's
+#     content. The hook runs AFTER sgdisk re-cuts the partition but BEFORE the
+#     `blkid` guard, so it erases the stale signature at the FINAL offset;
+#     `blkid` then comes back empty and `mkfs` actually runs.
 #
 # ⚠️  This disk is WIPED on install. This is the Kingston SA400 SSD that
 #     currently holds CachyOS (btrfs root+subvols on sdb2, ESP on sdb1).
@@ -34,6 +48,8 @@
             format = "vfat";
             mountpoint = "/boot";
             mountOptions = [ "umask=0077" ];
+            # erase any stale signature before disko's blkid format-guard (above)
+            preCreateHook = ''wipefs --all --force "$device"'';
           };
         };
         root = {
@@ -42,6 +58,9 @@
             type = "btrfs";
             extraArgs = [ "-f" ];
             mountpoint = "/";
+            # erase the stale CachyOS btrfs superblock before disko's blkid
+            # format-guard, otherwise mkfs.btrfs is skipped (see header comment)
+            preCreateHook = ''wipefs --all --force "$device"'';
           };
         };
       };