diff --git a/ocaml/Qmcchem_dataserver.ml b/ocaml/Qmcchem_dataserver.ml
index 11d965b..90199c2 100644
--- a/ocaml/Qmcchem_dataserver.ml
+++ b/ocaml/Qmcchem_dataserver.ml
@@ -392,7 +392,10 @@ let run ?(daemon=true) ezfio_filename =
     Unix.rename block_channel_filename_locked block_channel_filename_tmp;
     Random_variable.compress_files ();
     send_log "status" 0 t0 "Compressed block file";
-    block_channel := open_out_gen [ Open_append ] 0o660 block_channel_filename_locked
+    if Sys.file_exists block_channel_filename_locked then
+      block_channel := open_out_gen [ Open_append ] 0o660 block_channel_filename_locked
+    else
+      block_channel := open_out block_channel_filename_locked
   in
 
 
diff --git a/ocaml/Random_variable.ml b/ocaml/Random_variable.ml
index 14351c8..0eeab92 100644
--- a/ocaml/Random_variable.ml
+++ b/ocaml/Random_variable.ml
@@ -681,14 +681,15 @@ let compress_files () =
   let out_channel_dir = 
     let rand_num = Random.int 1000000 |> string_of_int in
     let tmp_dir =
-      Filename.concat "!Ezfio.ezfio_filename" @@
-      Filename.concat "blocks" @@
-      Filename.concat "qmc" rand_num
+      Filename.concat !Ezfio.ezfio_filename @@
+      Filename.concat "blocks" "qmc"^rand_num
     in
     try
       Unix.mkdir tmp_dir 0o755;
       tmp_dir
-    with _ -> raise (Sys_error "Cannot create temp dir")
+    with _ ->
+    let message = Printf.sprintf "Cannot create temp dir %s" tmp_dir in
+    raise (Sys_error message)
   in
 
   let out_channel_name =