diff --git a/Lib/zipfile.py b/Lib/zipfile.py
index 2476717..048f60a 100644
--- a/Lib/zipfile.py
+++ b/Lib/zipfile.py
@@ -1028,11 +1028,12 @@
                 # set the modified flag so central directory gets written
                 # even if no files are added to the archive
                 self._didModify = True
+                self._start_disk = 0
                 try:
-                    self.start_dir = self._start_disk = self.fp.tell()
+                    self.start_dir = self.fp.tell()
                 except (AttributeError, OSError):
                     self.fp = _Tellable(self.fp)
-                    self.start_dir = self._start_disk = 0
+                    self.start_dir = 0
                     self._seekable = False
                 else:
                     # Some file-like objects can provide tell() but not seek()
diff --git a/Misc/NEWS b/Misc/NEWS
index 0dd4ef1..ad67b1d 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -140,6 +140,9 @@
 Library
 -------
 
+- Issue #29094: Offsets in a ZIP file created with extern file object and modes
+  "w" and "x" now are relative to the start of the file.
+
 - Issue #13051: Fixed recursion errors in large or resized
   curses.textpad.Textbox.  Based on patch by Tycho Andersen.