diff --git a/src/cal_tools/files.py b/src/cal_tools/files.py
index 57beac95aab6571ce89ad407df818ffabb6b440f..5c82f953f064d41c5728475b2ef9bd97e99154ee 100644
--- a/src/cal_tools/files.py
+++ b/src/cal_tools/files.py
@@ -323,14 +323,19 @@ class DataFile(h5py.File):
         md_group.create_dataset('dataSources/dataSourceId',
                                 shape=data_sources_shape,
                                 data=[f'{sources[name]}/{name}'.encode('ascii')
-                                      for name in source_names])
+                                      for name in source_names],
+                                maxshape=(None,),
+                                )
         md_group.create_dataset('dataSources/deviceId',
                                 shape=data_sources_shape,
                                 data=[name.encode('ascii')
-                                      for name in source_names])
+                                      for name in source_names],
+                                maxshape=(None,))
         md_group.create_dataset('dataSources/root', shape=data_sources_shape,
                                 data=[sources[name].encode('ascii')
-                                      for name in source_names])
+                                      for name in source_names],
+                                maxshape=(None,),
+                                )
 
         md_group.create_dataset(
             'karaboFramework', shape=(1,),