diff --git a/src/borg/platform/base.py b/src/borg/platform/base.py
index e8d28b52d..51689db25 100644
--- a/src/borg/platform/base.py
+++ b/src/borg/platform/base.py
@@ -75,7 +75,7 @@ class SyncFile:
     """
 
     def __init__(self, path):
-        self.fd = open(path, 'wb')
+        self.fd = open(path, 'xb')
         self.fileno = self.fd.fileno()
 
     def __enter__(self):