diff --git a/colorizer_data/converter.py b/colorizer_data/converter.py
index 9d3005c..df44b01 100644
--- a/colorizer_data/converter.py
+++ b/colorizer_data/converter.py
@@ -443,7 +443,8 @@ def convert_colorizer_data(
             be flattened along the Z-axis using a max projection. If `None`, 2D frame generation
             will be skipped.
         frames_3d (Frames3dMetadata | None): A `Frames3dMetadata` object containing the 3D image source
-            ("source") and channel ("segmentation_channel") to use for the 3D image source.
+            ("source") and channel ("segmentation_channel") to use for the 3D image source. The source
+            should be the path to or the URL of an OME-Zarr array (preferred) or OME-TIFF file.
         centroid_x_column (str): The name of the column containing x-coordinates of object
             centroids, in pixels relative to the frame image, where 0 is the left edge of the
             image. Defaults to "Centroid X."
diff --git a/colorizer_data/types.py b/colorizer_data/types.py
index 2c3e893..151f52f 100644
--- a/colorizer_data/types.py
+++ b/colorizer_data/types.py
@@ -128,9 +128,13 @@ class Backdrop3dMetadata(DataClassJsonMixin):
     name: str
     source: str
     """
-    HTTPS or local path to an OME-Zarr source volume (e.g. ends with
-    `.ome.zarr`). Can be the same source as the segmentations defined in
-    `Frames3dMetadata`.
+    HTTPS URL or relative path from the dataset directory to an OME-Zarr source
+    volume (e.g. ends with `.ome.zarr`). Can be the same source as the
+    segmentations defined in `Frames3dMetadata`.
+    
+    Example: 
+     - `volumes/sample_data.ome.zarr`
+     - `https://example.com/data/sample_data.ome.zarr`
     """
     channel_index: str
     """
@@ -155,8 +159,12 @@ class Frames3dMetadata(DataClassJsonMixin):
 
     source: str
     """
-    HTTPS or local path to 3D data, ideally in OME-Zarr format (e.g. ends with
-    `.ome.zarr`).
+    HTTPS URL or relative path from the dataset directory to 3D data, ideally in
+    OME-Zarr format (e.g. ends with `.ome.zarr`).
+
+    Example: 
+     - `volumes/sample_data.ome.zarr`
+     - `https://example.com/data/sample_data.ome.zarr`
     """
     segmentation_channel: int = 0
     """The channel of segmentation data. `0` by default."""
diff --git a/colorizer_data/utils.py b/colorizer_data/utils.py
index 18fc6f1..aba30dc 100644
--- a/colorizer_data/utils.py
+++ b/colorizer_data/utils.py
@@ -776,3 +776,34 @@ def _get_frame_count_from_3d_source(source: str) -> int:
     # Attempt to read the image to get info (such as length)
     img = BioImage(source)
     return int(img.dims.T)
+
+
+def is_url(source: str) -> bool:
+    """
+    Checks if a source string is an HTTP(S) URL.
+    """
+    return source.startswith("http://") or source.startswith("https://")
+
+
+def check_file_source(name: str, source: str | None, outpath: pathlib.Path):
+    """
+    Logs warnings for missing or unreachable file sources.
+    """
+    if source is None:
+        logging.error(
+            f"{name} is undefined and will fail to load. Please provide a relative path inside the dataset directory or an HTTPS URL to an OME-Zarr (preferred) or OME-TIFF file."
+        )
+    elif not is_url(source):
+        # Check for absolute paths, parent paths, or missing files/folders.
+        if os.path.isabs(source):
+            logging.error(
+                f"{name} cannot be an absolute path and will fail to load. Please provide a relative path inside the dataset directory or an HTTPS URL. Received: '{source}'"
+            )
+        elif ".." in pathlib.Path(source).parts:
+            logging.warning(
+                f"{name} should not contain parent directory references ('..'), as it may fail to load in certain deploy environments. Received: '{source}'"
+            )
+        elif not os.path.exists(outpath / source):
+            logging.warning(
+                f"{name} path could not be found. Please check that it exists. Received: '{source}'"
+            )
diff --git a/colorizer_data/writer.py b/colorizer_data/writer.py
index 086610f..3a62929 100644
--- a/colorizer_data/writer.py
+++ b/colorizer_data/writer.py
@@ -22,6 +22,7 @@
     DEFAULT_FRAME_PREFIX,
     DEFAULT_FRAME_SUFFIX,
     _get_frame_count_from_3d_source,
+    check_file_source,
     cast_feature_to_info_type,
     copy_remote_or_local_file,
     generate_frame_paths,
@@ -711,3 +712,16 @@ def validate_dataset(
                     + " or add an offset if your frame numbers do not start at 0."
                     + " You may also need to generate the list of frames yourself if your dataset is skipping frames."
                 )
+
+        # Check that frames3d sources are reachable
+        if "frames3d" in self.manifest:
+            frames3d_metadata = Frames3dMetadata.from_dict(self.manifest["frames3d"])
+            source = frames3d_metadata.source
+            check_file_source("3D frames source", source, self.outpath)
+            # Validate backdrops
+            if frames3d_metadata.backdrops is not None:
+                for i in range(len(frames3d_metadata.backdrops)):
+                    backdrop_source = frames3d_metadata.backdrops[i].source
+                    check_file_source(
+                        f"3D frames backdrop {i} source", backdrop_source, self.outpath
+                    )