Adapt sync-sd-backup.sh to eMMC-primary R5C layout

The eMMC is now the boot/root device and the SD card is a manual
failsafe. U-Boot has no persistent env and always prefers an SD with a
boot.scr, so the SD's boot script is kept dormant as
boot-failesafe-rename-to:boot.scr.

- Locate the SD by root UUID instead of /dev/mmcblk0p2
- Preserve the SD's dormant boot script; never create a live boot.scr
  and refresh the dormant copy from the eMMC
- Copy fstab and armbianEnv.txt with the root UUID rewritten to the SD's
- Add rsync -x; drop the Pi-specific /boot/firmware exclude

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

failover/sync-sd-backup.sh

@@ -1,26 +1,39 @@
 #!/usr/bin/env bash
-# Refresh the SD card (mmcblk0p2) as a live boot-fallback clone of the
+# Refresh the SD card as a manual boot-fallback clone of the running root
-# running root filesystem. Run manually after a push:
+# filesystem. Run manually after a push:
 #   sudo ./failover/sync-sd-backup.sh
 #
-# EEPROM BOOT_ORDER falls back to the SD card if the primary boot disk
+# R5C layout: the primary root is the eMMC (mmcblk1p1) and the SD card
-# (NVMe) doesn't come up. /etc/fstab, /etc/machine-id, and /var/log are
+# (root UUID below) is the backup. U-Boot has no persistent env and always
-# excluded from the sync so the clone keeps its own partition identity
+# prefers the SD if it finds a boot.scr there, so the SD's boot.scr is kept
-# (PARTUUIDs differ per disk) and boot/journal history instead of
+# renamed to /boot/boot-failesafe-rename-to:boot.scr. U-Boot then skips the
-# inheriting the source disk's — see failover/README or repo memory
+# SD and boots the eMMC. To boot the SD: rename it back to boot.scr (e.g.
-# "sd-fallback-fstab-clobber" for what breaks if those leak across.
+# from another machine with the SD mounted, or from a U-Boot console).
+#
+# Not copied verbatim, so the clone keeps its own identity:
+#   /etc/fstab, /boot/armbianEnv.txt  copied with the root UUID rewritten
+#   /boot/boot.scr                    never created; the eMMC's copy is
+#                                     refreshed under the failsafe name
+#   /etc/machine-id, /var/log         left alone (see repo memory
+#                                     "sd-fallback-fstab-clobber")
+# The bootloader in the SD's first sectors (idbloader/u-boot) is not touched.
 set -euo pipefail
 
 [ "$(id -u)" -eq 0 ] || { echo "Run as root (sudo)." >&2; exit 1; }
 
-SD_ROOT_PART=/dev/mmcblk0p2
+SD_UUID=39118305-4927-47a4-acc7-ba022f37af22
+SD_ROOT_PART=/dev/disk/by-uuid/$SD_UUID
 MOUNT_POINT=/mnt/sd-backup-root
+FAILSAFE='boot-failesafe-rename-to:boot.scr'
+
+[ -e "$SD_ROOT_PART" ] || { echo "SD card (UUID $SD_UUID) not present." >&2; exit 1; }
 
 root_src=$(findmnt -no SOURCE /)
-if [ "$root_src" = "$SD_ROOT_PART" ]; then
+if [ "$(readlink -f "$SD_ROOT_PART")" = "$root_src" ]; then
-    echo "Currently booted from $SD_ROOT_PART — refusing to sync SD onto itself." >&2
+    echo "Currently booted from the SD card ($root_src) — refusing to sync SD onto itself." >&2
     exit 1
 fi
+root_uuid=$(findmnt -no UUID /)
 
 mkdir -p "$MOUNT_POINT"
 mounted_here=0
@@ -41,13 +54,16 @@ trap cleanup EXIT
 # rsync exit 24 ("partial transfer due to vanished source files") is expected on a
 # live root — files like container shm sockets or in-progress logs routinely disappear
 # mid-sync. Tolerate that one code; anything else is a real failure and still aborts.
+# Excluded paths are also protected from --delete (no --delete-excluded), which is
+# what keeps the SD's failsafe-renamed boot script in place.
 rc=0
-rsync -aHAX --numeric-ids --delete \
+rsync -aHAXx --numeric-ids --delete \
     --info=progress2 \
     --exclude=/proc/* --exclude=/sys/* --exclude=/dev/* --exclude=/run/* \
     --exclude=/tmp/* --exclude=/mnt/* --exclude=/media/* --exclude=/lost+found \
-    --exclude=/boot/firmware/* --exclude=/swapfile \
+    --exclude=/swapfile \
     --exclude=/etc/fstab --exclude=/etc/machine-id --exclude=/var/log/* \
+    --exclude=/boot/armbianEnv.txt --exclude=/boot/boot.scr --exclude="/boot/$FAILSAFE" \
     --stats / "$MOUNT_POINT/" || rc=$?
 
 if [ "$rc" -ne 0 ] && [ "$rc" -ne 24 ]; then
@@ -58,4 +74,18 @@ if [ "$rc" -eq 24 ]; then
     echo "Note: some files vanished mid-sync (rsync exit 24) — normal on a live root." >&2
 fi
 
+# fstab and armbianEnv.txt: take the source's content, point root at the SD.
+for f in etc/fstab boot/armbianEnv.txt; do
+    sed "s/$root_uuid/$SD_UUID/g" "/$f" > "$MOUNT_POINT/$f.new"
+    chmod --reference="/$f" "$MOUNT_POINT/$f.new"
+    mv "$MOUNT_POINT/$f.new" "$MOUNT_POINT/$f"
+done
+
+# Keep the SD's boot script current but dormant, so U-Boot never boots the SD
+# while the eMMC is healthy.
+install -m 0644 /boot/boot.scr "$MOUNT_POINT/boot/$FAILSAFE"
+if [ -e "$MOUNT_POINT/boot/boot.scr" ]; then
+    echo "WARNING: $MOUNT_POINT/boot/boot.scr exists — U-Boot will boot the SD, not the eMMC." >&2
+fi
+
 echo "SD backup ($SD_ROOT_PART) synced from $root_src."