diff --git a/spoolman/env.py b/spoolman/env.py
index 315faaf4..2220d123 100644
--- a/spoolman/env.py
+++ b/spoolman/env.py
@@ -4,6 +4,7 @@ import logging
 import os
 import subprocess
 import sys
+import tempfile
 from datetime import datetime
 from enum import Enum
 from pathlib import Path
@@ -458,9 +459,9 @@ def get_build_date() -> datetime | None:
 def can_write_to_data_dir() -> bool:
     """Check if the data directory is writable."""
     try:
-        test_file = get_data_dir().joinpath("test.txt")
-        test_file.touch()
-        test_file.unlink()
+        # A uniquely named file, so the check can never clobber one of the user's own.
+        with tempfile.NamedTemporaryFile(dir=get_data_dir()):
+            pass
     except:  # noqa: E722
         return False
     return True
diff --git a/tests/test_env.py b/tests/test_env.py
index 0a8d524b..17c0ba28 100644
--- a/tests/test_env.py
+++ b/tests/test_env.py
@@ -1,5 +1,8 @@
 """Tests for environment variable parsing."""
 
+import os
+from pathlib import Path
+
 import pytest
 
 from spoolman import env
@@ -46,3 +49,23 @@ def test_get_cors_origin_drops_empty_and_duplicate_entries(monkeypatch: pytest.M
 def test_get_cors_origin_raw_is_unparsed(monkeypatch: pytest.MonkeyPatch):
     monkeypatch.setenv("SPOOLMAN_CORS_ORIGIN", " https://a.local, https://b.local ")
     assert env.get_cors_origin_raw() == " https://a.local, https://b.local "
+
+
+def test_can_write_to_data_dir_leaves_existing_files_alone(monkeypatch: pytest.MonkeyPatch, tmp_path: Path):
+    """The write check must not touch a file of the user's, even one named like a probe file."""
+    monkeypatch.setenv("SPOOLMAN_DIR_DATA", str(tmp_path))
+    (tmp_path / "test.txt").write_text("mine")
+    assert env.can_write_to_data_dir() is True
+    assert [p.name for p in tmp_path.iterdir()] == ["test.txt"]
+    assert (tmp_path / "test.txt").read_text() == "mine"
+
+
+def test_can_write_to_data_dir_read_only(monkeypatch: pytest.MonkeyPatch, tmp_path: Path):
+    if os.name == "nt" or os.geteuid() == 0:
+        pytest.skip("directory permissions are not enforced here")
+    monkeypatch.setenv("SPOOLMAN_DIR_DATA", str(tmp_path))
+    tmp_path.chmod(0o500)
+    try:
+        assert env.can_write_to_data_dir() is False
+    finally:
+        tmp_path.chmod(0o700)