diff --git a/tests/conftest.py b/tests/conftest.py
index 08a0ada6..bebf3967 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import errno
 import os
 from typing import TYPE_CHECKING
 
@@ -85,6 +86,13 @@ def _isolate_git_config(
     monkeypatch.setenv("GIT_COMMITTER_EMAIL", "tests@example.com")
 
 
+# Errors that mean "this name is not representable here", as opposed to a
+# filesystem that is full, read-only, or otherwise broken.
+_REJECTED_NAME_ERRNOS = frozenset(
+    {errno.EINVAL, errno.EILSEQ, errno.ENAMETOOLONG},
+)
+
+
 @pytest.fixture
 def write_control_character_file() -> Callable[[Path, str, str], Path]:
     """Create a file whose *name* carries terminal control characters, or skip.
@@ -103,6 +111,11 @@ def write_control_character_file() -> Callable[[Path, str, str], Path]:
         try:
             path.write_text(content, encoding="utf-8")
         except OSError as exc:  # pragma: no cover - platform dependent
+            # Only "the name itself is unacceptable" is a reason to skip. A
+            # full disk or an unwritable temp directory must still fail these
+            # security tests rather than quietly reporting them as skipped.
+            if exc.errno not in _REJECTED_NAME_ERRNOS:
+                raise
             pytest.skip(f"this filesystem rejects control characters in filenames: {exc}")
         return path
 
diff --git a/tests/test_local_sources.py b/tests/test_local_sources.py
index f03bed46..be73fa84 100644
--- a/tests/test_local_sources.py
+++ b/tests/test_local_sources.py
@@ -182,9 +182,14 @@ def test_infer_target_type_applies_the_mount_policy() -> None:
     # so pick a protected system directory this platform actually has. The
     # policy itself already knows about both families (`_FORBIDDEN_MOUNT_TREES`
     # and `_FORBIDDEN_WINDOWS_TREE_NAMES`); only the fixture was Unix-only.
-    candidates = (
-        [Path(Path.cwd().anchor) / "Windows"] if os.name == "nt" else [Path("/etc"), Path("/usr")]
-    )
+    if os.name == "nt":
+        # Ask Windows where it is installed. Deriving the drive from the
+        # checkout would look for D:\Windows on a repo cloned to D:, miss, and
+        # skip the very branch this test exists to cover.
+        system_root = os.environ.get("SYSTEMROOT") or os.environ.get("WINDIR")
+        candidates = [Path(system_root)] if system_root else []
+    else:
+        candidates = [Path("/etc"), Path("/usr")]
     system_dir = next((p for p in candidates if p.is_dir()), None)
     if system_dir is None:
         pytest.skip("no protected system directory on this platform")