diff --git a/scripts/container-specs.sh b/scripts/container-specs.sh
index 54ea0fa5..3bfc0852 100755
--- a/scripts/container-specs.sh
+++ b/scripts/container-specs.sh
@@ -26,7 +26,15 @@ done
 
 # ── Environment detection ─────────────────────────────────────────────
 detect_environment() {
-  DISK_GB=$(df --output=size -BG / 2>/dev/null | tail -1 | tr -dc '0-9')
+  # Measure disk where container data actually lives, not the OS partition.
+  # Archipelago installs mount a separate (usually-encrypted) data volume at
+  # /var/lib/archipelago on any host with meaningful storage, so checking /
+  # would always report the ~30 GB OS partition and wrongly trip prune mode
+  # on 2 TB boxes. Fall back to / only for first-boot before the data
+  # partition is mounted.
+  local disk_target="/var/lib/archipelago"
+  [ -d "$disk_target" ] || disk_target="/"
+  DISK_GB=$(df --output=size -BG "$disk_target" 2>/dev/null | tail -1 | tr -dc '0-9')
   DISK_GB=${DISK_GB:-500}
   TOTAL_MEM_MB=$(($(awk '/MemTotal/{print $2}' /proc/meminfo 2>/dev/null || echo 16000000) / 1024))
   LOW_MEM=false