diff --git a/CHANGELOG.md b/CHANGELOG.md
index f8bb7dd..c26a5cc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,13 @@
 
 All notable changes to this project will be documented in this file.
 
+## [unreleased]
+
+### 🧪 Testing
+
+- *(fs)* Update test descriptions and comments to English - ([c4f4614](https://git.0xmax42.io/maxp/systemd-timer/commit/c4f4614a2daee68f9b33b9676106214c65a1a427))
+- *(fs)* Add rollback tests for writeUnitFiles errors - ([6039d23](https://git.0xmax42.io/maxp/systemd-timer/commit/6039d236eb7de449ce22b1d9ea718389a3e2261d))
+
 ## [0.4.1](https://git.0xmax42.io/maxp/systemd-timer/compare/v0.4.0..v0.4.1) - 2025-05-28
 
 ### 🐛 Bug Fixes
diff --git a/deno.lock b/deno.lock
index a182b0a..4bf34d8 100644
--- a/deno.lock
+++ b/deno.lock
@@ -168,7 +168,8 @@
     "https://deno.land/std@0.224.0/path/windows/relative.ts": "3e1abc7977ee6cc0db2730d1f9cb38be87b0ce4806759d271a70e4997fc638d7",
     "https://deno.land/std@0.224.0/path/windows/resolve.ts": "8dae1dadfed9d46ff46cc337c9525c0c7d959fb400a6308f34595c45bdca1972",
     "https://deno.land/std@0.224.0/path/windows/to_file_url.ts": "40e560ee4854fe5a3d4d12976cef2f4e8914125c81b11f1108e127934ced502e",
-    "https://deno.land/std@0.224.0/path/windows/to_namespaced_path.ts": "4ffa4fb6fae321448d5fe810b3ca741d84df4d7897e61ee29be961a6aac89a4c"
+    "https://deno.land/std@0.224.0/path/windows/to_namespaced_path.ts": "4ffa4fb6fae321448d5fe810b3ca741d84df4d7897e61ee29be961a6aac89a4c",
+    "https://deno.land/std@0.224.0/testing/mock.ts": "a963181c2860b6ba3eb60e08b62c164d33cf5da7cd445893499b2efda20074db"
   },
   "workspace": {
     "dependencies": [
diff --git a/src/utils/__tests__/fs.test.ts b/src/utils/__tests__/fs.test.ts
index 5109a54..d0fa3c6 100644
--- a/src/utils/__tests__/fs.test.ts
+++ b/src/utils/__tests__/fs.test.ts
@@ -3,11 +3,13 @@ import {
     assertExists,
     assertStringIncludes,
 } from 'https://deno.land/std@0.224.0/assert/mod.ts';
+import { stub } from 'https://deno.land/std@0.224.0/testing/mock.ts';
+import { exists } from 'https://deno.land/std@0.224.0/fs/mod.ts';
 import { join } from 'https://deno.land/std@0.224.0/path/mod.ts';
 import { resolveUnitTargetPath, writeUnitFiles } from '../mod.ts';
 import { TimerOptions } from '../../types/options.ts';
 
-Deno.test('writeUnitFiles schreibt .service und .timer korrekt', async () => {
+Deno.test('writeUnitFiles: writes .service and .timer files correctly', async () => {
     const tmp = await Deno.makeTempDir({ prefix: 'test-units-' });
 
     const options = {
@@ -26,15 +28,15 @@ Deno.test('writeUnitFiles schreibt .service und .timer korrekt', async () => {
         options as TimerOptions,
     ) as { servicePath: string; timerPath: string };
 
-    // Überprüfe Pfade
+    // Check file paths
     assertEquals(servicePath, join(tmp, 'test-backup.service'));
     assertEquals(timerPath, join(tmp, 'test-backup.timer'));
 
-    // Existieren Dateien?
+    // Check if files exist
     assertExists(await Deno.stat(servicePath));
     assertExists(await Deno.stat(timerPath));
 
-    // Enthält die Datei den erwarteten Inhalt?
+    // Check if file contents match expectations
     const readService = await Deno.readTextFile(servicePath);
     const readTimer = await Deno.readTextFile(timerPath);
 
@@ -42,18 +44,237 @@ Deno.test('writeUnitFiles schreibt .service und .timer korrekt', async () => {
     assertStringIncludes(readTimer, 'OnCalendar=daily');
 });
 
-Deno.test('resolveUnitTargetPath mit --output', () => {
+Deno.test('resolveUnitTargetPath: with --output', () => {
     const result = resolveUnitTargetPath({ output: '/tmp/units', user: false });
     assertEquals(result, '/tmp/units');
 });
 
-Deno.test('resolveUnitTargetPath mit --user ohne output', () => {
+Deno.test('resolveUnitTargetPath: with --user and no output', () => {
     Deno.env.set('HOME', '/home/maxp');
     const result = resolveUnitTargetPath({ output: undefined, user: true });
     assertEquals(result, '/home/maxp/.config/systemd/user');
 });
 
-Deno.test('resolveUnitTargetPath ohne output und ohne user', () => {
+Deno.test('resolveUnitTargetPath: with no output and no user', () => {
     const result = resolveUnitTargetPath({ output: undefined, user: false });
     assertEquals(result, '/etc/systemd/system');
 });
+
+Deno.test('writeUnitFiles: error writing .timer file triggers rollback', async () => {
+    const tmp = await Deno.makeTempDir();
+
+    const options: TimerOptions = {
+        output: tmp,
+        user: false,
+        exec: '/bin/true',
+        calendar: 'never',
+    };
+
+    const name = 'fail-timer';
+    const serviceContent = '[Service]\nExecStart=/bin/true';
+    const timerContent = '[Timer]\nOnCalendar=never';
+
+    const servicePath = join(tmp, `${name}.service`);
+    const timerPath = join(tmp, `${name}.timer`);
+
+    // Simulate: writing the .timer file fails
+    const originalWrite = Deno.writeTextFile;
+    const writeStub = stub(
+        Deno,
+        'writeTextFile',
+        async (path: string | URL, data: string | ReadableStream<string>) => {
+            if (typeof path === 'string' && path.endsWith('.timer')) {
+                throw new Error('Simulated write error');
+            } else {
+                return await originalWrite(path, data);
+            }
+        },
+    );
+
+    const result = await writeUnitFiles(
+        name,
+        serviceContent,
+        timerContent,
+        options,
+    );
+
+    // Expect: function returns undefined
+    assertEquals(result, undefined);
+
+    // Expect: both files have been deleted (rollback)
+    assertEquals(await exists(servicePath), false);
+    assertEquals(await exists(timerPath), false);
+
+    writeStub.restore();
+});
+
+Deno.test('writeUnitFiles: error writing .service file prevents further actions', async () => {
+    const tmp = await Deno.makeTempDir();
+
+    const options: TimerOptions = {
+        output: tmp,
+        user: false,
+        exec: '/bin/true',
+        calendar: 'weekly',
+    };
+
+    const name = 'fail-service';
+    const serviceContent = '[Service]\nExecStart=/bin/true';
+    const timerContent = '[Timer]\nOnCalendar=weekly';
+
+    const servicePath = join(tmp, `${name}.service`);
+    const timerPath = join(tmp, `${name}.timer`);
+
+    // Simulate: error writing the .service file
+    const writeStub = stub(
+        Deno,
+        'writeTextFile',
+        (path: string | URL, _data: string | ReadableStream<string>) => {
+            if (typeof path === 'string' && path.endsWith('.service')) {
+                throw new Error('Simulated service write error');
+            }
+            return Promise.resolve();
+        },
+    );
+
+    const result = await writeUnitFiles(
+        name,
+        serviceContent,
+        timerContent,
+        options,
+    );
+
+    // Expect: function returns undefined
+    assertEquals(result, undefined);
+
+    // Expect: no files were created
+    assertEquals(await exists(servicePath), false);
+    assertEquals(await exists(timerPath), false);
+
+    writeStub.restore();
+});
+
+Deno.test('writeUnitFiles: both files written, then error → full rollback', async () => {
+    const tmp = await Deno.makeTempDir();
+
+    const options: TimerOptions = {
+        output: tmp,
+        user: false,
+        exec: '/bin/true',
+        calendar: 'never',
+    };
+
+    const name = 'fail-after-write';
+    const serviceContent = '[Service]\nExecStart=/bin/true';
+    const timerContent = '[Timer]\nOnCalendar=never';
+
+    const servicePath = join(tmp, `${name}.service`);
+    const timerPath = join(tmp, `${name}.timer`);
+
+    let writeCount = 0;
+    const originalWriteTextFile = Deno.writeTextFile;
+
+    const writeStub = stub(
+        Deno,
+        'writeTextFile',
+        async (path: string | URL, data: string | ReadableStream<string>) => {
+            if (typeof path !== 'string') {
+                throw new Error('Unexpected path type');
+            }
+
+            // Simulate both writes, then throw an error after the second
+            writeCount++;
+
+            await originalWriteTextFile(path, data);
+
+            if (writeCount === 2) {
+                throw new Error('Simulated error after full write');
+            }
+        },
+    );
+
+    const result = await writeUnitFiles(
+        name,
+        serviceContent,
+        timerContent,
+        options,
+    );
+
+    // Expect: function returns undefined
+    assertEquals(result, undefined);
+
+    // Expect: both files were removed
+    assertEquals(await exists(servicePath), false);
+    assertEquals(await exists(timerPath), false);
+
+    writeStub.restore();
+});
+
+Deno.test('writeUnitFiles: rollback fails if files cannot be deleted', async () => {
+    const tmp = await Deno.makeTempDir();
+
+    const options: TimerOptions = {
+        output: tmp,
+        user: false,
+        exec: '/bin/true',
+        calendar: 'never',
+    };
+
+    const name = 'rollback-error';
+    const serviceContent = '[Service]\nExecStart=/bin/true';
+    const timerContent = '[Timer]\nOnCalendar=never';
+
+    const servicePath = join(tmp, `${name}.service`);
+    const timerPath = join(tmp, `${name}.timer`);
+
+    const originalWriteTextFile = Deno.writeTextFile;
+    const _originalRemove = Deno.remove;
+
+    let writeCount = 0;
+
+    const writeStub = stub(
+        Deno,
+        'writeTextFile',
+        async (path: string | URL, data: string | ReadableStream<string>) => {
+            writeCount++;
+            await originalWriteTextFile(path, data);
+            if (writeCount === 2) {
+                throw new Error('Error after full write');
+            }
+        },
+    );
+
+    const removeStub = stub(
+        Deno,
+        'remove',
+        // deno-lint-ignore require-await
+        async (_path: string | URL, _opts?: Deno.RemoveOptions) => {
+            throw new Error('Deletion forbidden!');
+        },
+    );
+
+    const logs: string[] = [];
+    const consoleStub = stub(console, 'error', (...args) => {
+        logs.push(args.map((a) => String(a)).join(' '));
+    });
+
+    const result = await writeUnitFiles(
+        name,
+        serviceContent,
+        timerContent,
+        options,
+    );
+    assertEquals(result, undefined);
+
+    // Files still exist because deletion failed
+    assertEquals(await exists(servicePath), true);
+    assertEquals(await exists(timerPath), true);
+
+    // Error output contains "rollback_failed"
+    const combinedLogs = logs.join('\n');
+    assertStringIncludes(combinedLogs, 'rollback_failed');
+
+    writeStub.restore();
+    removeStub.restore();
+    consoleStub.restore();
+});