diff --git a/build/fbcode_builder/getdeps/buildopts.py b/build/fbcode_builder/getdeps/buildopts.py
index 2bc73d9a1..70d3662b2 100644
--- a/build/fbcode_builder/getdeps/buildopts.py
+++ b/build/fbcode_builder/getdeps/buildopts.py
@@ -10,6 +10,7 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 import base64
 import errno
 import hashlib
+import ntpath
 import os
 import subprocess
 import tempfile
@@ -178,11 +179,14 @@ def list_win32_subst_letters():
     return mapping
 
 
-def find_existing_win32_subst_for_path(path):
-    path = os.path.normpath(path)
-    mapping = list_win32_subst_letters()
-    for letter, target in mapping.items():
-        if target == path:
+def find_existing_win32_subst_for_path(
+    path,  # type: str
+    subst_mapping,  # type: typing.Mapping[str, str]
+):
+    # type: (...) -> typing.Optional[str]
+    path = ntpath.normcase(ntpath.normpath(path))
+    for letter, target in subst_mapping.items():
+        if ntpath.normcase(target) == path:
             return letter
     return None
 
@@ -210,7 +214,9 @@ def find_unused_drive_letter():
 
 def create_subst_path(path):
     for _attempt in range(0, 24):
-        drive = find_existing_win32_subst_for_path(path)
+        drive = find_existing_win32_subst_for_path(
+            path, subst_mapping=list_win32_subst_letters()
+        )
         if drive:
             return drive
         available = find_unused_drive_letter()
diff --git a/build/fbcode_builder/getdeps/test/scratch_test.py b/build/fbcode_builder/getdeps/test/scratch_test.py
new file mode 100644
index 000000000..fd8f4819e
--- /dev/null
+++ b/build/fbcode_builder/getdeps/test/scratch_test.py
@@ -0,0 +1,83 @@
+#!/usr/bin/env python
+# Copyright (c) 2019-present, Facebook, Inc.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree. An additional grant
+# of patent rights can be found in the PATENTS file in the same directory.
+
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import unittest
+
+from ..buildopts import find_existing_win32_subst_for_path
+
+
+class Win32SubstTest(unittest.TestCase):
+    def test_no_existing_subst(self):
+        self.assertIsNone(
+            find_existing_win32_subst_for_path(
+                r"C:\users\alice\appdata\local\temp\fbcode_builder_getdeps",
+                subst_mapping={},
+            )
+        )
+        self.assertIsNone(
+            find_existing_win32_subst_for_path(
+                r"C:\users\alice\appdata\local\temp\fbcode_builder_getdeps",
+                subst_mapping={"X:\\": r"C:\users\alice\appdata\local\temp\other"},
+            )
+        )
+
+    def test_exact_match_returns_drive_path(self):
+        self.assertEqual(
+            find_existing_win32_subst_for_path(
+                r"C:\temp\fbcode_builder_getdeps",
+                subst_mapping={"X:\\": r"C:\temp\fbcode_builder_getdeps"},
+            ),
+            "X:\\",
+        )
+        self.assertEqual(
+            find_existing_win32_subst_for_path(
+                r"C:/temp/fbcode_builder_getdeps",
+                subst_mapping={"X:\\": r"C:/temp/fbcode_builder_getdeps"},
+            ),
+            "X:\\",
+        )
+
+    def test_multiple_exact_matches_returns_arbitrary_drive_path(self):
+        self.assertIn(
+            find_existing_win32_subst_for_path(
+                r"C:\temp\fbcode_builder_getdeps",
+                subst_mapping={
+                    "X:\\": r"C:\temp\fbcode_builder_getdeps",
+                    "Y:\\": r"C:\temp\fbcode_builder_getdeps",
+                    "Z:\\": r"C:\temp\fbcode_builder_getdeps",
+                },
+            ),
+            ("X:\\", "Y:\\", "Z:\\"),
+        )
+
+    def test_drive_letter_is_case_insensitive(self):
+        self.assertEqual(
+            find_existing_win32_subst_for_path(
+                r"C:\temp\fbcode_builder_getdeps",
+                subst_mapping={"X:\\": r"c:\temp\fbcode_builder_getdeps"},
+            ),
+            "X:\\",
+        )
+
+    def test_path_components_are_case_insensitive(self):
+        self.assertEqual(
+            find_existing_win32_subst_for_path(
+                r"C:\TEMP\FBCODE_builder_getdeps",
+                subst_mapping={"X:\\": r"C:\temp\fbcode_builder_getdeps"},
+            ),
+            "X:\\",
+        )
+        self.assertEqual(
+            find_existing_win32_subst_for_path(
+                r"C:\temp\fbcode_builder_getdeps",
+                subst_mapping={"X:\\": r"C:\TEMP\FBCODE_builder_getdeps"},
+            ),
+            "X:\\",
+        )