diff --git a/gnommo/cli.py b/gnommo/cli.py
index 0c2e0d6..b6dab04 100644
--- a/gnommo/cli.py
+++ b/gnommo/cli.py
@@ -4520,6 +4520,23 @@ def cmd_render(
 
     # Check for unaligned markers
     unaligned = [t for t in marker_timings if t.timestamp < 0]
+    if slide_range and unaligned:
+        # Partial (--slides) render: only unaligned markers INSIDE the requested
+        # range should block it — failures elsewhere in the manuscript are
+        # irrelevant to this window. Restrict to the manuscript-order span
+        # [start_slide, end_slide].
+        order = [t.marker_id for t in marker_timings]
+        start_slide, end_slide = slide_range
+        lo = order.index(start_slide) if start_slide in order else 0
+        hi = order.index(end_slide) if (end_slide and end_slide in order) else len(order) - 1
+        in_range = set(order[lo : hi + 1])
+        skipped = [t for t in unaligned if t.marker_id not in in_range]
+        unaligned = [t for t in unaligned if t.marker_id in in_range]
+        if skipped:
+            print(
+                f"\n  ({len(skipped)} unaligned marker(s) outside the {start_slide}:"
+                f"{end_slide or ''} range ignored for this partial render)"
+            )
     if unaligned:
         print(f"\n  WARNING: {len(unaligned)} marker(s) could not be aligned!")
         for t in unaligned:
diff --git a/gnommo/parser.py b/gnommo/parser.py
index 333409b..f79229f 100644
--- a/gnommo/parser.py
+++ b/gnommo/parser.py
@@ -476,11 +476,12 @@ def parse_videos(
 
     videos = {}
     for video_id, video_data in data.items():
+        
         if "source_file" not in video_data:
             raise ParseError(
                 f"Video '{video_id}' missing required field 'source_file'", videos_path
             )
-
+        
         # Parse attribution if present
         attribution = None
         if "attribution" in video_data:
diff --git a/gnommo/preprocessor.py b/gnommo/preprocessor.py
index 4972cda..9c55450 100644
--- a/gnommo/preprocessor.py
+++ b/gnommo/preprocessor.py
@@ -806,7 +806,7 @@ def preprocess_video(
                 filter_type=first_filter_type,
             )
 
-    # If output_file is specified, copy/rename to final location and clean up
+    # If output_file is specified, move/copy to final location and clean up
     if video_source.output_file:
         import shutil
 
@@ -814,11 +814,32 @@ def preprocess_video(
 
         final_output.parent.mkdir(parents=True, exist_ok=True)
 
-        # Write to a .tmp file first so that an interrupted copy never leaves a
-        # partial file at the final path (which would be silently skipped next run).
+        # Stage on a .tmp on the destination filesystem, then atomically swap in —
+        # an interrupted transfer never leaves a partial file at the final path.
         tmp_output = final_output.with_suffix(".tmp")
-        shutil.copy2(current_input, tmp_output)
-        tmp_output.replace(final_output)  # atomic on same filesystem
+
+        # current_input is the last batch's scratch intermediate (about to be
+        # deleted anyway), so MOVE it into place rather than copying — concat has
+        # already written this multi-GB ProRes once, and a rename is instant on the
+        # same filesystem. Only fall back to a copy across filesystems (e.g. scratch
+        # on an external SSD, output on the internal disk). Never move the original
+        # source (transcribe-only path leaves current_input == source).
+        moved = False
+        if current_input in intermediate_files:
+            try:
+                os.replace(current_input, tmp_output)
+                moved = True
+            except OSError:
+                moved = False  # cross-filesystem — copy instead
+        if not moved:
+            try:
+                size_gb = current_input.stat().st_size / (1024**3)
+                print(f"        Finalizing: copying {size_gb:.1f} GB to output (cross-disk)...")
+            except OSError:
+                pass
+            shutil.copy2(current_input, tmp_output)
+
+        os.replace(tmp_output, final_output)  # atomic on the destination filesystem
 
         if verbose:
             print(f"    Final output: {final_output}")