diff --git a/src/transcription/ui/runtime_settings_store.py b/src/transcription/ui/runtime_settings_store.py
index b45aa15..2b4a881 100644
--- a/src/transcription/ui/runtime_settings_store.py
+++ b/src/transcription/ui/runtime_settings_store.py
@@ -6,6 +6,9 @@ import os
 import re
 import tempfile
 from dataclasses import dataclass
+from errno import EBUSY
+from errno import EPERM
+from errno import EXDEV
 from pathlib import Path
 from typing import Literal
 
@@ -518,6 +521,7 @@ def _upsert_env_key(lines: list[str], key: str, value: str) -> None:
 
 
 def _write_env_lines_atomic(*, path: Path, lines: list[str]) -> None:
+    temp_path: Path | None = None
     try:
         path.parent.mkdir(parents=True, exist_ok=True)
         content = "\n".join(lines).rstrip("\n")
@@ -533,7 +537,15 @@ def _write_env_lines_atomic(*, path: Path, lines: list[str]) -> None:
         ) as handle:
             temp_path = Path(handle.name)
             handle.write(content)
-        temp_path.replace(path)
+        try:
+            temp_path.replace(path)
+        except OSError as exc:
+            # Single-file bind mounts can reject replace() (cross-device or busy mountpoint).
+            # Fallback to direct write so Runtime Settings can persist to mounted env files.
+            if exc.errno not in {EXDEV, EBUSY, EPERM}:
+                raise
+            with path.open("w", encoding="utf-8", newline="\n") as handle:
+                handle.write(content)
     except OSError as exc:
         raise AppError(
             "Runtime settings file is not writable.",
@@ -541,6 +553,9 @@ def _write_env_lines_atomic(*, path: Path, lines: list[str]) -> None:
             suggestion="Verify file path and write permissions, then retry.",
             detail=f"Failed writing runtime env file {path}: {type(exc).__name__}: {exc}",
         ) from exc
+    finally:
+        if temp_path is not None:
+            temp_path.unlink(missing_ok=True)
 
 
 def _escape_json_string(value: str) -> str:
diff --git a/tests/ui/test_runtime_settings_store.py b/tests/ui/test_runtime_settings_store.py
index e669434..77281ab 100644
--- a/tests/ui/test_runtime_settings_store.py
+++ b/tests/ui/test_runtime_settings_store.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import errno
 from pathlib import Path
 
 import pytest
@@ -132,3 +133,18 @@ def test_save_runtime_settings_surfaces_unwritable_target(tmp_path: Path, monkey
     with pytest.raises(AppError) as exc:
         save_runtime_settings(settings=settings, updates={"port": "9001"}, env_file_path=env_path)
     assert exc.value.message == "Runtime settings file is not writable."
+
+
+def test_save_runtime_settings_falls_back_when_atomic_replace_is_unavailable(tmp_path: Path, monkeypatch):
+    settings = _settings_for_runtime_editing(tmp_path)
+    env_path = tmp_path / ".env.production"
+    env_path.write_text("OPENROUTER_API_KEY=test-key\nPORT=8000\n", encoding="utf-8")
+
+    def _replace_cross_device(_self: Path, _target: Path) -> Path:
+        raise OSError(errno.EXDEV, "Invalid cross-device link")
+
+    monkeypatch.setattr(Path, "replace", _replace_cross_device)
+    snapshot = save_runtime_settings(settings=settings, updates={"port": "9001"}, env_file_path=env_path)
+
+    assert snapshot.env_file_path == env_path
+    assert "PORT=9001" in env_path.read_text(encoding="utf-8")