diff --git a/syconn/handler/basics.py b/syconn/handler/basics.py
index 54cd38fc90af25160b507b7f3c5ec7169490c920..8c962a046849f38cb82690e2af9dc1ece2b38df8 100644
--- a/syconn/handler/basics.py
+++ b/syconn/handler/basics.py
@@ -366,7 +366,7 @@ def write_data2kzip(kzip_path, fpath, fname_in_zip=None, force_overwrite=False):
     data2kzip(kzip_path, [fpath], [fname_in_zip], force_overwrite)
 
 
-def data2kzip(kzip_path, fpaths, fnames_in_zip=None, force_overwrite=False,
+def data2kzip(kzip_path, fpaths, fnames_in_zip=None, force_overwrite=True,
               verbose=False):
     """
     Write files to k.zip.
@@ -380,6 +380,8 @@ def data2kzip(kzip_path, fpaths, fnames_in_zip=None, force_overwrite=False,
     verbose : bool
     force_overwrite : bool
     """
+    if not force_overwrite:
+        raise NotImplementedError('Currently modification of data in existing kzip is not implemented.')
     nb_files = len(fpaths)
     if verbose:
         log_handler.info('Writing {} files to .zip.'.format(nb_files))
diff --git a/syconn/proc/meshes.py b/syconn/proc/meshes.py
index 73879228ef845534db87c5d8b9f9621c8dbb6182..da02ccf2362bb740c968b65cb48b7f4fa3502e66 100644
--- a/syconn/proc/meshes.py
+++ b/syconn/proc/meshes.py
@@ -757,7 +757,7 @@ def write_mesh2kzip(k_path, ind, vert, norm, color, ply_fname,
 
 
 def write_meshes2kzip(k_path, inds, verts, norms, colors, ply_fnames,
-                      force_overwrite=False, verbose=True):
+                      force_overwrite=True, verbose=True):
     """
     Writes meshes as .ply's to k.zip file.
 
@@ -774,6 +774,8 @@ def write_meshes2kzip(k_path, inds, verts, norms, colors, ply_fnames,
     force_overwrite : bool
     verbose : bool
     """
+    if not force_overwrite:
+        raise NotImplementedError('Currently modification of data in existing kzip is not implemented.')
     tmp_paths = []
     if verbose:
         log_proc.info('Generating ply files.')