diff --git a/install.ps1 b/install.ps1
index 98b0ed7d..7903d8ff 100644
--- a/install.ps1
+++ b/install.ps1
@@ -8,7 +8,7 @@ param(
 
 $ErrorActionPreference = "Stop"
 $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-$defaultInstallSha256 = "1a10ac215c60706699d267fcfa31c4947d6aa792b46455e078a5861fa8c5e2ea"
+$defaultInstallSha256 = "16732df165f2f3cf971ae2049aea4b89c7b6a48925d395417c9a363e9bb57970"
 $maximumInstallerBytes = 2MB
 $installerExit = 0
 $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
diff --git a/install.py b/install.py
index 66908857..364d8105 100644
--- a/install.py
+++ b/install.py
@@ -2604,6 +2604,17 @@ def _claim_partial(path: Path) -> BinaryIO | None:
     except OSError:
         handle.close()
         return None
+    # Opened and locked are two steps: in between, the run that held the
+    # partial may have renamed it into place and released it. The lock then
+    # sits on the installed BIOS, which looks like a finished partial and
+    # would be truncated. Only the file still under the partial's name is ours.
+    try:
+        ours = os.path.samestat(os.fstat(handle.fileno()), os.lstat(path))
+    except OSError:
+        ours = False
+    if not ours:
+        handle.close()
+        return None
     return handle
 
 
diff --git a/install.sh b/install.sh
index 6957c00a..640614fb 100755
--- a/install.sh
+++ b/install.sh
@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="1a10ac215c60706699d267fcfa31c4947d6aa792b46455e078a5861fa8c5e2ea"
+DEFAULT_INSTALL_SHA256="16732df165f2f3cf971ae2049aea4b89c7b6a48925d395417c9a363e9bb57970"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/tests/test_install_experience.py b/tests/test_install_experience.py
index 9eb27c3d..cb257561 100644
--- a/tests/test_install_experience.py
+++ b/tests/test_install_experience.py
@@ -145,6 +145,31 @@ class TwoRunsOneFolder(unittest.TestCase):
             self.assertIsNotNone(install._claim_partial(part))
 
 
+class APartialRenamedBeforeTheLock(unittest.TestCase):
+    """Open and lock are two steps. The run holding the partial could finish
+    in between, rename it into place and release it: the second run then
+    locked the installed BIOS, took it for a finished partial and truncated it."""
+
+    @unittest.skipIf(os.name == "nt", "flock semantics")
+    def test_the_installed_file_is_not_claimed(self):
+        import fcntl
+
+        with tempfile.TemporaryDirectory() as tmp:
+            part = Path(tmp) / ".big.bin.part"
+            final = Path(tmp) / "big.bin"
+            part.write_bytes(PAYLOAD)
+            real_flock = fcntl.flock
+
+            def finish_first(fd, op):
+                os.replace(part, final)
+                return real_flock(fd, op)
+
+            with mock.patch.object(fcntl, "flock", finish_first):
+                claimed = install._claim_partial(part)
+            self.assertIsNone(claimed)
+            self.assertEqual(final.read_bytes(), PAYLOAD)
+
+
 class InstalledFileModes(unittest.TestCase):
     """A file came out 0600, the mode of the temporary it was written to: run
     with sudo for a root-owned BIOS folder, the emulator's own user could not