diff --git a/system_setup/install.sh b/system_setup/install.sh
index 658a156..bc95dd3 100755
--- a/system_setup/install.sh
+++ b/system_setup/install.sh
@@ -345,14 +345,14 @@ if [[ "$MODE" == "shrink" ]]; then
     echo "  After shrink      : ${CURR_START} MiB – ${NEW_END} MiB  (filesystem: ${NEW_FS_MIB} MiB)"
     echo "  New OEMDRV        : ${OEMDRV_START} MiB – ${OEMDRV_END} MiB  (${SHRINK_MIB} MiB)"
 
-    if [[ "$S_MNT" != "-" && -n "$S_MNT" ]]; then
-        info "Unmounting $S_DEV from $S_MNT..."
-        ORIG_MNT="$S_MNT"
-        umount "$S_DEV" || die "Cannot unmount $S_DEV. Close all open files and try again."
-        WAS_MOUNTED=1
-    fi
-
     if [[ "$S_FS" == "ext4" ]]; then
+        # ext4 requires offline resize — unmount first
+        if [[ "$S_MNT" != "-" && -n "$S_MNT" ]]; then
+            info "Unmounting $S_DEV from $S_MNT..."
+            ORIG_MNT="$S_MNT"
+            umount "$S_DEV" || die "Cannot unmount $S_DEV. Close all open files and try again."
+            WAS_MOUNTED=1
+        fi
         info "Running e2fsck on $S_DEV..."
         e2fsck -f "$S_DEV" || die "e2fsck found errors on $S_DEV. Fix them before retrying."
 
@@ -360,13 +360,20 @@ if [[ "$MODE" == "shrink" ]]; then
         resize2fs "$S_DEV" "${NEW_FS_MIB}M" || die "resize2fs failed."
 
     elif [[ "$S_FS" == "btrfs" ]]; then
+        # btrfs supports online resize — use existing mount point if available,
+        # otherwise mount temporarily
         info "Shrinking btrfs filesystem on $S_DEV to ${NEW_FS_MIB} MiB..."
-        TMP_MNT=$(mktemp -d /tmp/oemdrv_btrfs.XXXXXX)
-        mount "$S_DEV" "$TMP_MNT" || { rmdir "$TMP_MNT"; die "Cannot mount $S_DEV for btrfs resize."; }
-        btrfs filesystem resize "${NEW_FS_MIB}m" "$TMP_MNT" \
-            || { umount "$TMP_MNT"; rmdir "$TMP_MNT"; die "btrfs filesystem resize failed."; }
-        sync
-        umount "$TMP_MNT" && rmdir "$TMP_MNT"
+        if [[ "$S_MNT" != "-" && -n "$S_MNT" ]]; then
+            btrfs filesystem resize "${NEW_FS_MIB}m" "$S_MNT" \
+                || die "btrfs filesystem resize failed."
+        else
+            TMP_MNT=$(mktemp -d /tmp/oemdrv_btrfs.XXXXXX)
+            mount "$S_DEV" "$TMP_MNT" || { rmdir "$TMP_MNT"; die "Cannot mount $S_DEV for btrfs resize."; }
+            btrfs filesystem resize "${NEW_FS_MIB}m" "$TMP_MNT" \
+                || { umount "$TMP_MNT"; rmdir "$TMP_MNT"; die "btrfs filesystem resize failed."; }
+            sync
+            umount "$TMP_MNT" && rmdir "$TMP_MNT"
+        fi
     fi
 
     info "Shrinking partition $S_PNUM to ${NEW_END} MiB in partition table..."