diff --git a/scripts/install-usb.sh b/scripts/install-usb.sh
index d70cd07..f3ee020 100755
--- a/scripts/install-usb.sh
+++ b/scripts/install-usb.sh
@@ -60,7 +60,15 @@ trap cleanup EXIT
 mkdir -p "$TMP/boot" "$TMP/data"
 mount "$p2" "$TMP/boot"; mount "$p3" "$TMP/data"
 mkdir -p "$TMP/boot/boot/grub" "$TMP/data/reports" "$TMP/data/database" "$TMP/data/checklists" "$TMP/data/reference"
-cp "$ISO" "$TMP/boot/pc-intake.iso"
+if ! cp "$ISO" "$TMP/boot/pc-intake.iso" || ! sync -f "$TMP/boot"; then
+  current_size=$(blockdev --getsize64 "$DEVICE" 2>/dev/null || echo 0)
+  if ((current_size == 0)); then
+    echo 'USB medium disappeared while writing. Replace the stick or use another USB port.' >&2
+  else
+    echo 'USB write failed. Check the stick and the kernel log before retrying.' >&2
+  fi
+  exit 1
+fi
 # Copy video reference files separately under INTAKE_DATA/reference (optional).
 cat > "$TMP/boot/boot/grub/grub.cfg" <<'GRUBCFG'
 set timeout=5