diff --git a/docs/architecture/backup-dr.md b/docs/architecture/backup-dr.md
index 9fa30480..84ae792c 100644
--- a/docs/architecture/backup-dr.md
+++ b/docs/architecture/backup-dr.md
@@ -333,7 +333,9 @@ Two-step offsite sync:
 **Change tracking**: `nfs-change-tracker.service` (systemd, inotifywait) on PVE host watches `/srv/nfs` and `/srv/nfs-ssd` continuously. Changed file paths are logged to `/mnt/backup/.nfs-changes.log`. The offsite sync reads this log and transfers only changed files. Incremental syncs complete in seconds instead of 30+ minutes.
 **Monthly full sync**: On 1st Sunday of month, runs `rsync --delete` for cleanup (removes orphaned files on Synology).
 
-**Path exclusions**: `/srv/nfs/anca-elements/` (~770G) is excluded from both layers — it is itself a downstream replica of `Synology:/volume1/Backup/Anca/Elements` (synced in via `anca-elements-sync.sh`), so backing it up to Synology would be a self-duplicate. The exclusion lives in `nfs-change-tracker.service` (inotify `--exclude` regex) and `offsite-sync-backup` (rsync `--exclude` on full sync + `grep -v` on the incremental files-from list).
+**Path exclusions**: `/srv/nfs/anca-elements/` (~770G) is excluded from both layers. From 2026-05-24 onward `/srv/nfs/anca-elements` is the source of truth for this archive — the Synology copy at `/volume1/Backup/Anca/Elements` was deleted (it had been the upstream source, but anca-elements-sync.sh's role inverted: PVE now writes, Synology no longer holds it). Single-disk-failure protection is provided by a SEPARATE local mirror on sda (`anca-elements-mirror.{service,timer}`, weekly Mon 04:00) — not by Synology. The Synology exclusion lives in `nfs-change-tracker.service` (inotify `--exclude` regex) and `offsite-sync-backup` (rsync `--exclude` on full sync + `grep -v` on the incremental files-from list).
+
+**Layer 3a: anca-elements local mirror (sda)**: `/usr/local/bin/anca-elements-mirror` rsyncs `/srv/nfs/anca-elements/` → `/mnt/backup/anca-elements/` weekly. `rsync -rlt --delete -H --no-perms --no-owner --no-group`. Idempotent; subsequent runs only transfer changes. Pushes `anca_elements_mirror_last_run_timestamp` + `anca_elements_mirror_last_status` to Pushgateway. No offsite copy — by design; the archive is single-disk-failure tolerant only.
 
 **Destination**:
 - `Synology/Backup/Viki/nfs/` — mirrors `/srv/nfs`
@@ -360,6 +362,8 @@ Two-step offsite sync:
 | `/etc/systemd/system/lvm-pvc-snapshot.timer` | Daily 03:00 (LVM snapshots) |
 | `/etc/systemd/system/daily-backup.timer` | Daily 05:00 (file backup) |
 | `/etc/systemd/system/offsite-sync-backup.timer` | Daily 06:00 (offsite sync) |
+| `/usr/local/bin/anca-elements-mirror` | PVE host: weekly mirror of /srv/nfs/anca-elements → sda /mnt/backup/anca-elements |
+| `/etc/systemd/system/anca-elements-mirror.timer` | Weekly Mon 04:00 (anca-elements mirror) |
 | `stacks/dbaas/` | Terraform: PostgreSQL/MySQL backup CronJobs |
 | `stacks/vault/` | Terraform: Vault backup CronJob |
 | `stacks/vaultwarden/` | Terraform: Vaultwarden backup + integrity CronJobs |
diff --git a/scripts/anca-elements-mirror.service b/scripts/anca-elements-mirror.service
new file mode 100644
index 00000000..db1bf270
--- /dev/null
+++ b/scripts/anca-elements-mirror.service
@@ -0,0 +1,15 @@
+[Unit]
+Description=Mirror /srv/nfs/anca-elements to /mnt/backup (single-disk-failure protection)
+After=network-online.target local-fs.target
+Wants=network-online.target
+
+[Service]
+Type=oneshot
+ExecStart=/usr/local/bin/anca-elements-mirror
+StandardOutput=journal
+StandardError=journal
+SyslogIdentifier=anca-elements-mirror
+# Big sustained IO — don't compete with foreground services.
+Nice=10
+IOSchedulingClass=idle
+TimeoutStartSec=18000
diff --git a/scripts/anca-elements-mirror.sh b/scripts/anca-elements-mirror.sh
new file mode 100644
index 00000000..4ce61ca2
--- /dev/null
+++ b/scripts/anca-elements-mirror.sh
@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+# anca-elements-mirror — single-disk-failure mirror of /srv/nfs/anca-elements → /mnt/backup
+#
+# Deploy to PVE host at /usr/local/bin/anca-elements-mirror.
+# Schedule: weekly Mon 04:00 via systemd timer (anca-elements-mirror.timer).
+#
+# WHY: /srv/nfs/anca-elements lives on the sdc thin pool. Synology no longer
+# holds the original (deleted after this mirror was verified). sda /mnt/backup
+# is the only other local disk with room (~770G) — this gives us a single-
+# disk-failure copy. No offsite for this archive (intentional, see backup-dr.md).
+#
+# Idempotent: `rsync -aH --delete` makes destination match source exactly.
+# Re-runs only transfer changed files.
+
+set -euo pipefail
+
+SRC=/srv/nfs/anca-elements
+DST=/mnt/backup/anca-elements
+LOG=/var/log/anca-elements-mirror.log
+LOCKFILE=/run/anca-elements-mirror.lock
+PUSHGATEWAY="${ANCA_MIRROR_PUSHGATEWAY:-http://10.0.20.100:30091}"
+PUSHGATEWAY_JOB=anca-elements-mirror
+
+log()  { echo "[$(date -u '+%Y-%m-%dT%H:%M:%SZ')] $*" | tee -a "$LOG"; }
+warn() { log "WARN: $*"; }
+
+push_metrics() {
+    local status="${1:-0}" bytes="${2:-0}"
+    cat <<EOF | curl -s --connect-timeout 5 --max-time 10 --data-binary @- "${PUSHGATEWAY}/metrics/job/${PUSHGATEWAY_JOB}" 2>/dev/null || true
+anca_elements_mirror_last_run_timestamp $(date +%s)
+anca_elements_mirror_last_status ${status}
+anca_elements_mirror_bytes ${bytes}
+EOF
+}
+
+KILLED=""
+cleanup() {
+    rm -f "$LOCKFILE"
+    if [ -n "$KILLED" ]; then
+        push_metrics 2 0  # status=2 → aborted (matches lvm-pvc-snapshot convention)
+    fi
+}
+trap cleanup EXIT
+trap 'KILLED=1; exit 143' TERM INT
+
+if ! ( set -o noclobber; echo $$ > "$LOCKFILE" ) 2>/dev/null; then
+    log "FATAL: another instance running (pid $(cat "$LOCKFILE" 2>/dev/null || echo unknown))"
+    exit 1
+fi
+
+mountpoint -q /mnt/backup || { log "FATAL: /mnt/backup not mounted"; push_metrics 1 0; exit 1; }
+[ -d "$SRC" ]              || { log "FATAL: source $SRC missing"; push_metrics 1 0; exit 1; }
+
+mkdir -p "$DST"
+
+log "=== mirror starting: $SRC → $DST ==="
+SRC_SIZE_GB=$(du -sBG "$SRC" 2>/dev/null | awk '{print $1}')
+log "source size: $SRC_SIZE_GB"
+
+# -aH preserves hardlinks (probably none here, cheap insurance).
+# --info=stats2 emits a final transfer summary into the log.
+# --no-perms / --no-owner / --no-group: source has root:www-data 2775 and
+# we don't need to perfectly preserve those on the mirror copy — dest will
+# inherit /mnt/backup's defaults. (Symmetric with anca-elements-sync.sh's
+# choice when copying FROM Synology.)
+RSYNC_RC=0
+rsync \
+    -rlt --delete -H \
+    --no-perms --no-owner --no-group \
+    --info=stats2 \
+    "$SRC/" "$DST/" 2>&1 | tee -a "$LOG" || RSYNC_RC=${PIPESTATUS[0]}
+
+DST_BYTES=$(du -sb "$DST" 2>/dev/null | awk '{print $1}')
+
+if [ "$RSYNC_RC" -eq 0 ]; then
+    log "=== mirror complete; dest size: $(du -sh "$DST" | cut -f1) ==="
+    push_metrics 0 "$DST_BYTES"
+else
+    log "=== mirror failed: rsync exited $RSYNC_RC ==="
+    push_metrics 1 "$DST_BYTES"
+    exit "$RSYNC_RC"
+fi
diff --git a/scripts/anca-elements-mirror.timer b/scripts/anca-elements-mirror.timer
new file mode 100644
index 00000000..642a7773
--- /dev/null
+++ b/scripts/anca-elements-mirror.timer
@@ -0,0 +1,10 @@
+[Unit]
+Description=Weekly anca-elements mirror to /mnt/backup
+
+[Timer]
+OnCalendar=Mon *-*-* 04:00:00
+Persistent=true
+RandomizedDelaySec=15min
+
+[Install]
+WantedBy=timers.target