diff --git a/lib/upload-audit.js b/lib/upload-audit.js
index eade08c..7234d42 100644
--- a/lib/upload-audit.js
+++ b/lib/upload-audit.js
@@ -42,7 +42,7 @@ function createInternalLogWriter(options) {
   const maxBackups = Number.isFinite(source.maxBackups) ? source.maxBackups : 2;
   let activePath = null;
 
-  if (!fs || !fs.promises || typeof fs.promises.appendFile !== 'function' || typeof resolveInternalLogPath !== 'function' || typeof fileName !== 'string') {
+  if (!fs || !fs.promises || typeof fs.promises.appendFile !== 'function' || typeof fs.appendFileSync !== 'function' || typeof resolveInternalLogPath !== 'function' || typeof fileName !== 'string') {
     throw new TypeError('createInternalLogWriter requires fs, fileName and resolveInternalLogPath');
   }
 
@@ -66,8 +66,36 @@ function createInternalLogWriter(options) {
     return false;
   }
 
+  function appendSync(line, label) {
+    const excludedPaths = new Set();
+    for (let attempt = 0; attempt < retryDelays.length; attempt++) {
+      const targetPath = resolveInternalLogPath(fileName, excludedPaths);
+      if (!targetPath) break;
+      excludedPaths.add(targetPath);
+      try {
+        fs.mkdirSync(path.dirname(targetPath), { recursive: true });
+        rotateLogFile(targetPath, maxBytes, maxBackups);
+        fs.appendFileSync(targetPath, line, 'utf-8');
+        activePath = targetPath;
+        return true;
+      } catch (error) {
+        reportError(label, error);
+      }
+    }
+    return false;
+  }
+
+  function flushSync(buffer, label) {
+    if (!Array.isArray(buffer) || buffer.length === 0) return true;
+    if (!appendSync(buffer.join(''), label)) return false;
+    buffer.length = 0;
+    return true;
+  }
+
   return {
     append,
+    appendSync,
+    flushSync,
     getActivePath: () => activePath,
     getPath: () => activePath || resolveInternalLogPath(fileName)
   };
diff --git a/main.js b/main.js
index 3ddff45..6911d2e 100644
--- a/main.js
+++ b/main.js
@@ -1785,8 +1785,7 @@ app.on('will-quit', () => {
   } catch {}
   try {
     if (_rotLogBuffer.length) {
-      fs.appendFileSync(getRotLogPath(), _rotLogBuffer.join(''), 'utf-8');
-      _rotLogBuffer.length = 0;
+      _rotLogWriter.flushSync(_rotLogBuffer, 'rot-log');
     }
   } catch {}
 });
diff --git a/tests/upload-audit.test.js b/tests/upload-audit.test.js
index 41add76..84266c9 100644
--- a/tests/upload-audit.test.js
+++ b/tests/upload-audit.test.js
@@ -55,6 +55,7 @@ test('internal rotation writer retries inside userData and reports the file that
   const rotationTargets = [];
   const testFs = {
     mkdirSync: fs.mkdirSync,
+    appendFileSync: fs.appendFileSync,
     promises: {
       appendFile: async (targetPath, ...args) => {
         appendTargets.push(targetPath);
@@ -82,6 +83,55 @@ test('internal rotation writer retries inside userData and reports the file that
   assert.equal(fs.readFileSync(fallbackPath, 'utf8'), '[rotation]\n');
 });
 
+test('synchronous rotation flush falls back completely and retains buffered lines when every target fails', (t) => {
+  const directory = createTempDirectory(t, 'mhu-internal-log-sync-writer-');
+  const userDataPath = path.join(directory, 'user-data');
+  const primaryPath = path.join(userDataPath, 'logs', 'account-rotation.log');
+  const fallbackPath = path.join(userDataPath, 'internal-logs', 'account-rotation.log');
+  fs.mkdirSync(primaryPath, { recursive: true });
+  const resolveInternalLogPath = createInternalLogPathResolver({ fs, path, userDataPath });
+  const writer = createInternalLogWriter({
+    fs,
+    path,
+    fileName: 'account-rotation.log',
+    resolveInternalLogPath,
+    rotateLogFile: () => {},
+    reportError: () => {}
+  });
+  const buffer = ['[rotation 1]\n', '[rotation 2]\n'];
+
+  assert.equal(writer.flushSync(buffer, 'rot-log'), true);
+  assert.deepEqual(buffer, []);
+  assert.equal(writer.getActivePath(), fallbackPath);
+  assert.equal(fs.readFileSync(fallbackPath, 'utf8'), '[rotation 1]\n[rotation 2]\n');
+
+  const failedUserDataPath = path.join(directory, 'failed-user-data');
+  const failedPrimaryPath = path.join(failedUserDataPath, 'logs', 'account-rotation.log');
+  const failedFallbackPath = path.join(failedUserDataPath, 'internal-logs', 'account-rotation.log');
+  fs.mkdirSync(failedPrimaryPath, { recursive: true });
+  fs.mkdirSync(failedFallbackPath, { recursive: true });
+  const failedWriter = createInternalLogWriter({
+    fs,
+    path,
+    fileName: 'account-rotation.log',
+    resolveInternalLogPath: createInternalLogPathResolver({ fs, path, userDataPath: failedUserDataPath }),
+    rotateLogFile: () => {},
+    reportError: () => {}
+  });
+  const retainedBuffer = ['[retained 1]\n', '[retained 2]\n'];
+
+  assert.equal(failedWriter.flushSync(retainedBuffer, 'rot-log'), false);
+  assert.deepEqual(retainedBuffer, ['[retained 1]\n', '[retained 2]\n']);
+  assert.equal(failedWriter.getActivePath(), null);
+});
+
+test('quit flush delegates the rotation buffer to the synchronous internal writer', () => {
+  const mainSource = fs.readFileSync(path.join(__dirname, '..', 'main.js'), 'utf8');
+
+  assert.match(mainSource, /_rotLogWriter\.flushSync\(_rotLogBuffer, 'rot-log'\);/);
+  assert.doesNotMatch(mainSource, /appendFileSync\(getRotLogPath\(\), _rotLogBuffer\.join\(''\)/);
+});
+
 test('upload audit writer leaves the configured fileuploader log contract unchanged', async (t) => {
   const directory = createTempDirectory(t, 'mhu-upload-log-contract-');
   const userDataPath = path.join(directory, 'user-data');