diff --git a/webapp/app.py b/webapp/app.py
index ad010ea..dbf5477 100644
--- a/webapp/app.py
+++ b/webapp/app.py
@@ -113,9 +113,9 @@ def images_import():
             src_items = os.listdir(source)
 
             # Move files from network upload to save disk space; copy from USB.
+            # deploy.sync_tree uses rsync --checksum: unchanged files are
+            # skipped, only new/changed files are written (no full replace).
             use_move = source == config.UPLOAD_DIR or source.startswith(config.UPLOAD_DIR + "/")
-            _transfer = shutil.move if use_move else shutil.copy2
-            _transfer_tree = shutil.move if use_move else shutil.copytree
 
             top_dirs = {d for d in src_items if os.path.isdir(os.path.join(source, d))}
             full_layout = "Deploy" in top_dirs
@@ -134,8 +134,7 @@ def images_import():
                     elif os.path.isdir(src_item) and item in shared_root:
                         prefix_key = target.split("-")[0] + "-"
                         shared_dest = os.path.join(config.SHARED_DIR, f"{prefix_key}{item}")
-                        os.makedirs(shared_dest, exist_ok=True)
-                        deploy._merge_tree(src_item, shared_dest, move=use_move)
+                        deploy.sync_tree(src_item, shared_dest, move=use_move)
                         dst_item = os.path.join(root, item)
                         if os.path.islink(dst_item):
                             os.remove(dst_item)
@@ -143,12 +142,9 @@ def images_import():
                             shutil.rmtree(dst_item)
                         os.symlink(shared_dest, dst_item)
                     elif os.path.isdir(src_item):
-                        dst_item = os.path.join(root, item)
-                        if os.path.exists(dst_item):
-                            shutil.rmtree(dst_item)
-                        _transfer_tree(src_item, dst_item)
+                        deploy.sync_tree(src_item, os.path.join(root, item), move=use_move)
                     else:
-                        _transfer(src_item, os.path.join(root, item))
+                        deploy.sync_tree(src_item, os.path.join(root, item), move=use_move)
             else:
                 deploy.import_deploy(source, dest, target, move=use_move)
 
diff --git a/webapp/services/deploy.py b/webapp/services/deploy.py
index 629758f..d7166de 100644
--- a/webapp/services/deploy.py
+++ b/webapp/services/deploy.py
@@ -7,6 +7,7 @@ re-use the same multi-GB driver tree without doubling disk usage.
 
 import os
 import shutil
+import subprocess
 
 import config
 from services.system import find_usb_mounts
@@ -21,27 +22,43 @@ def _replace_with_symlink(link_path, target_path):
     os.symlink(target_path, link_path)
 
 
-def _merge_tree(src, dst, move=False):
-    """Recursively merge src tree into dst, overwriting existing files.
+def sync_tree(src, dst, move=False, checksum=True):
+    """Copy src -> dst with rsync, SKIPPING files whose content already
+    matches and transferring only new or changed files.
 
-    When move=True, files are moved instead of copied (saves disk space
-    on imports from the local upload-dir).
-    """
-    _transfer = shutil.move if move else shutil.copy2
-    _transfer_tree = shutil.move if move else shutil.copytree
-    for item in os.listdir(src):
-        s = os.path.join(src, item)
-        d = os.path.join(dst, item)
-        if os.path.isdir(s):
-            if os.path.isdir(d):
-                _merge_tree(s, d, move=move)
-            else:
-                if os.path.exists(d):
-                    os.remove(d)
-                _transfer_tree(s, d)
-        else:
-            os.makedirs(os.path.dirname(d), exist_ok=True)
-            _transfer(s, d)
+    src may be a file or a directory (its contents merge into dst). With
+    checksum=True rsync compares by hash (--checksum), so a re-import only
+    rewrites files that actually differ instead of replacing everything;
+    set checksum=False for the faster size+mtime comparison. move=True
+    removes source files after a successful transfer (frees the upload dir).
+    Existing dst files not present in src are left untouched (merge, not
+    mirror - no --delete)."""
+    flags = ["-a"]                        # recurse, preserve perms/times/symlinks
+    if checksum:
+        flags.append("--checksum")        # compare by content hash, not size+mtime
+    if move:
+        flags.append("--remove-source-files")
+    if os.path.isdir(src):
+        os.makedirs(dst, exist_ok=True)
+        src_arg = src.rstrip("/") + "/"   # trailing slash = merge CONTENTS into dst
+    else:
+        parent = os.path.dirname(dst)
+        if parent:
+            os.makedirs(parent, exist_ok=True)
+        src_arg = src
+    subprocess.run(["rsync", *flags, src_arg, dst], check=True)
+    if move and os.path.isdir(src):
+        # --remove-source-files empties files but leaves the dir skeleton; prune it
+        for root, _dirs, _files in os.walk(src, topdown=False):
+            try:
+                os.rmdir(root)
+            except OSError:
+                pass
+
+
+def _merge_tree(src, dst, move=False):
+    """Backwards-compatible alias: checksum-aware merge of src into dst."""
+    sync_tree(src, dst, move=move)
 
 
 def import_deploy(src_deploy, dst_deploy, target="", move=False):