diff --git a/README.md b/README.md
index 5c2b697..9a26755 100644
--- a/README.md
+++ b/README.md
@@ -92,7 +92,7 @@ cp config/courses.example.json config/courses.json
 | `courses[].destination` | yes | Where the files land. Absolute, or relative to the repository root. No two courses may share one, or nest one inside another. |
 | `profilePath` | no | The saved browser session. Defaults to `.data/chrome-profile`. |
 | `statePath` | no | What has already been downloaded. Defaults to `.data/state.json`. |
-| `driveMountPath` | no (watchdog yes) | The Google Drive mount that contains the destinations. The watchdog writes no destination when this directory is absent. |
+| `driveMountPath` | no (watchdog yes) | The Google Drive mount that contains the destinations. Before writing, the watchdog requires both this directory and each destination's first Drive root below it to be present. |
 | `watchdogTimeoutMs` | no | The watchdog's initial timeout in milliseconds. The Owner pins the placeholder `900000` from the first week's logged durations. |
 | `media.mediaRoot` | required for `active`/`pilot` | The explicit Media store. It must be a directory below `/Volumes/RAID0`; there is no system-disk fallback. |
 | `media.freeSpaceReserveBytes` | no | Free space retained on the Media store before setup or acquisition. Defaults to 100 GiB. |
diff --git a/docs/adr/0013-the-watchdog-is-a-local-scheduled-two-layer-run.md b/docs/adr/0013-the-watchdog-is-a-local-scheduled-two-layer-run.md
index f59c09d..648a6d2 100644
--- a/docs/adr/0013-the-watchdog-is-a-local-scheduled-two-layer-run.md
+++ b/docs/adr/0013-the-watchdog-is-a-local-scheduled-two-layer-run.md
@@ -38,9 +38,11 @@ a second course-reading authority.
 - **Retrying reds.** Only a crash or timeout with no usable report is retried, three attempts total.
   A completed red is evidence the run understood and must remain visible under `docs/adr/0012`;
   retrying it would hammer NTULearn while hiding the signal that needs a person.
-- **Pre-checks beyond the Drive mount.** The mount check prevents a 05:00 run from creating a
-  phantom local destination. More guards can misfire, block a healthy run, and become a second
-  interpretation of the run's evidence, so they are not added without a concrete failure to solve.
+- **Pre-checks beyond the configured Drive roots.** The watchdog requires the mount and each
+  destination's first root below it to exist, preventing a 05:00 run from creating a phantom local
+  destination when `My Drive` moves but its former parent remains. More guards can misfire, block a
+  healthy run, and become a second interpretation of the run's evidence, so they are not added
+  without a concrete failure to solve.
 
 ## Consequences
 
diff --git a/src/watchdog/run.mjs b/src/watchdog/run.mjs
index c039fdf..000eec4 100644
--- a/src/watchdog/run.mjs
+++ b/src/watchdog/run.mjs
@@ -2,7 +2,7 @@ import { randomUUID } from "node:crypto";
 import { mkdir, readFile } from "node:fs/promises";
 import { clearTimeout, setTimeout as schedule } from "node:timers";
 import { setTimeout as sleep } from "node:timers/promises";
-import { dirname, join, relative, resolve } from "node:path";
+import { dirname, isAbsolute, join, relative, resolve, sep } from "node:path";
 import { INITIAL_WATCHDOG_TIMEOUT_MS } from "../config.mjs";
 import { isDirectoryPresent, writeAtomically } from "../sync/files.mjs";
 import { isCrashOrTimeout, sessionLapsed, watchdogVerdict } from "./verdict.mjs";
@@ -15,15 +15,15 @@ export async function runWatchdog({ config, root, runner }) {
   const paths = watchdogPaths(config.statePath);
   await mkdir(paths.stateDirectory, { recursive: true });
 
-  const preCheck = await driveMountCheck(config);
-  if (!preCheck.present) {
+  const preCheck = await destinationReadinessCheck(config);
+  if (!preCheck.ready) {
     const timestamp = new Date();
     const run = buildRun({
       startedAt: timestamp,
       finishedAt: timestamp,
       attempts: 0,
       timeoutMs: timeoutFor(config),
-      preChecks: preChecksFor(preCheck, [{ attempt: 0, driveMount: preCheck }]),
+      preChecks: preChecksFor(preCheck, [{ attempt: 0, ...preCheck }]),
       attemptResults: [],
       sync: null,
       verify: null,
@@ -50,11 +50,11 @@ export async function runWatchdogLocked({ config, root, runner, wait = sleep })
   let lastAttempt = { sync: null, verify: null };
 
   for (let attempt = 1; attempt <= MAX_ATTEMPTS; attempt += 1) {
-    const driveMount = await driveMountCheck(config);
-    preCheckHistory.push({ attempt, driveMount });
-    if (!driveMount.present) break;
+    const destinationReadiness = await destinationReadinessCheck(config);
+    preCheckHistory.push({ attempt, ...destinationReadiness });
+    if (!destinationReadiness.ready) break;
 
-    const result = await runAttempt({ root, runner, timeoutMs: timeoutFor(config) });
+    const result = await runAttempt({ config, root, runner, timeoutMs: timeoutFor(config) });
     lastAttempt = result;
     attemptResults.push({ attempt, ...result });
 
@@ -63,7 +63,7 @@ export async function runWatchdogLocked({ config, root, runner, wait = sleep })
   }
 
   const finishedAt = new Date();
-  const latestPreCheck = preCheckHistory.at(-1)?.driveMount ?? null;
+  const latestPreCheck = preCheckHistory.at(-1) ?? null;
   const run = buildRun({
     startedAt,
     finishedAt,
@@ -87,24 +87,91 @@ function watchdogPaths(statePath) {
   };
 }
 
-async function runAttempt({ root, runner, timeoutMs }) {
+async function runAttempt({ config, root, runner, timeoutMs }) {
   const timeoutAt = Date.now() + timeoutMs;
-  const sync = await captureCommand({ root, runner, command: "sync", timeoutAt });
+  const sync = classifyDestinationPermission(
+    await captureCommand({ root, runner, command: "sync", timeoutAt }),
+    config.courses ?? [],
+  );
   if (isCrashOrTimeout(sync)) return { sync, verify: null };
 
-  const verify = await captureCommand({ root, runner, command: "verify", timeoutAt });
+  const verify = classifyDestinationPermission(
+    await captureCommand({ root, runner, command: "verify", timeoutAt }),
+    config.courses ?? [],
+  );
   return { sync, verify };
 }
 
-async function driveMountCheck(config) {
+function classifyDestinationPermission(command, courses) {
+  const match = command.stderr?.match(/\b(EACCES|EPERM)\b[^\n]*?['"]([^'"]+)['"]/i);
+  if (!match) return command;
+
+  const deniedPath = resolve(match[2]);
+  const course = courses.find(({ destination }) => pathsOverlap(destination, deniedPath));
+  if (!course) return command;
   return {
-    path: config.driveMountPath,
-    present: Boolean(config.driveMountPath && (await isDirectoryPresent(config.driveMountPath))),
+    ...command,
+    destinationPermission: {
+      code: match[1].toUpperCase(),
+      destination: course.destination,
+      path: deniedPath,
+    },
   };
 }
 
-function preChecksFor(driveMount, history) {
-  return { driveMount, history };
+function pathsOverlap(left, right) {
+  const comparableLeft = resolve(left).toLowerCase();
+  const comparableRight = resolve(right).toLowerCase();
+  return (
+    comparableLeft === comparableRight ||
+    comparableLeft.startsWith(`${comparableRight}${sep}`) ||
+    comparableRight.startsWith(`${comparableLeft}${sep}`)
+  );
+}
+
+async function destinationReadinessCheck(config) {
+  const mountPresent = Boolean(
+    config.driveMountPath && (await isDirectoryPresent(config.driveMountPath)),
+  );
+  const destinations = mountPresent
+    ? await Promise.all(
+        (config.courses ?? []).map(async (course) => {
+          const root = destinationDriveRoot(config.driveMountPath, course.destination);
+          return {
+            path: course.destination,
+            root,
+            present:
+              destinationRelation(config.driveMountPath, course.destination).inside &&
+              (await isDirectoryPresent(root)),
+          };
+        }),
+      )
+    : [];
+  return {
+    ready: mountPresent && destinations.every((destination) => destination.present),
+    driveMount: { path: config.driveMountPath, present: mountPresent },
+    destinations,
+  };
+}
+
+function destinationDriveRoot(driveMountPath, destination) {
+  const { inside, pathFromMount } = destinationRelation(driveMountPath, destination);
+  if (!inside) return driveMountPath;
+  if (!pathFromMount) return driveMountPath;
+  return join(driveMountPath, pathFromMount.split(sep)[0]);
+}
+
+function destinationRelation(driveMountPath, destination) {
+  const pathFromMount = relative(driveMountPath, destination);
+  const inside =
+    !isAbsolute(pathFromMount) && pathFromMount !== ".." && !pathFromMount.startsWith(`..${sep}`);
+  return { inside, pathFromMount };
+}
+
+function preChecksFor(destinationReadiness, history) {
+  if (!destinationReadiness) return { history };
+  const { driveMount, destinations } = destinationReadiness;
+  return { driveMount, destinations, history };
 }
 
 async function runWithLock({ root, runner, lockPath }) {
@@ -249,6 +316,7 @@ function timeoutFor(config) {
 }
 
 function shouldRetry({ sync, verify }) {
+  if ([sync, verify].some((command) => command?.destinationPermission)) return false;
   if ([sync, verify].some(sessionLapsed)) return false;
   return [sync, verify].some(isCrashOrTimeout);
 }
diff --git a/src/watchdog/verdict.mjs b/src/watchdog/verdict.mjs
index 4ce6d50..c898262 100644
--- a/src/watchdog/verdict.mjs
+++ b/src/watchdog/verdict.mjs
@@ -15,6 +15,14 @@ export function watchdogVerdict({
     return { verdict: "yellow", message: "skipped: a run was already going" };
   }
 
+  const absentDestination = preChecks.destinations?.find((entry) => !entry.present);
+  if (absentDestination) {
+    return {
+      verdict: "red",
+      message: `Destination ${absentDestination.path} is unreachable — expected Drive root ${absentDestination.root}; set driveMountPath and the destination to the mounted Drive, then run: npm run watchdog`,
+    };
+  }
+
   if (preChecks.driveMount?.present === false) {
     return {
       verdict: "red",
@@ -23,6 +31,16 @@ export function watchdogVerdict({
     };
   }
 
+  const destinationPermission = [sync, verify].find(
+    (command) => command?.destinationPermission,
+  )?.destinationPermission;
+  if (destinationPermission) {
+    return {
+      verdict: "red",
+      message: `Destination ${destinationPermission.destination} is unreachable — permission denied at ${destinationPermission.path}; correct driveMountPath or destination permissions, then run: npm run watchdog`,
+    };
+  }
+
   if ([sync, verify].some(sessionLapsed)) {
     return { verdict: "red", message: "session lapsed — run `npm run login`" };
   }
diff --git a/test/watchdog-run.test.mjs b/test/watchdog-run.test.mjs
index 2bfc06b..d5f79c8 100644
--- a/test/watchdog-run.test.mjs
+++ b/test/watchdog-run.test.mjs
@@ -59,6 +59,40 @@ test("checks the Drive mount before acquiring the lock", async () => {
   );
 });
 
+test("refuses before writing when a destination's Drive root is absent", async () => {
+  const root = await mkdtemp(join(tmpdir(), "ntulearn-watchdog-"));
+  const driveMountPath = join(root, "Google Drive");
+  const driveRoot = join(driveMountPath, "My Drive");
+  const destination = join(driveRoot, "Modules", "AB1234", "NTULearn");
+  await mkdir(driveMountPath);
+  const config = {
+    statePath: join(root, ".data", "state.json"),
+    driveMountPath,
+    watchdogTimeoutMs: 1_000,
+    courses: [{ key: "AB1234", destination }],
+  };
+  const runner = {
+    spawn() {
+      throw new Error("the destination pre-check should stop before spawning a command");
+    },
+  };
+
+  const digest = await runWatchdogLocked({ config, root, runner });
+
+  assert.equal(digest.verdict, "red");
+  assert.equal(
+    digest.message,
+    `Destination ${destination} is unreachable — expected Drive root ${driveRoot}; set driveMountPath and the destination to the mounted Drive, then run: npm run watchdog`,
+  );
+  await assert.rejects(stat(destination), { code: "ENOENT" });
+
+  const run = JSON.parse(await readFile(join(root, ".data", digest.runLog), "utf8"));
+  assert.equal(run.attempts, 0);
+  assert.deepEqual(run.preChecks.destinations, [
+    { path: destination, root: driveRoot, present: false },
+  ]);
+});
+
 test("kills each timed-out attempt as a process group and retries three times", async () => {
   const root = await mkdtemp(join(tmpdir(), "ntulearn-watchdog-"));
   const driveMountPath = join(root, "Google Drive");
@@ -178,6 +212,49 @@ test("does not retry a lapsed session or a completed red run", async () => {
   assert.deepEqual(waits, []);
 });
 
+test("does not retry a destination permission failure", async () => {
+  const root = await mkdtemp(join(tmpdir(), "ntulearn-watchdog-"));
+  const driveMountPath = join(root, "Google Drive");
+  const driveRoot = join(driveMountPath, "My Drive");
+  const destination = join(driveRoot, "Modules", "AB1234", "NTULearn");
+  await mkdir(destination, { recursive: true });
+  const commands = [];
+  const waits = [];
+
+  const digest = await runWatchdogLocked({
+    config: {
+      statePath: join(root, ".data", "state.json"),
+      driveMountPath,
+      watchdogTimeoutMs: 1_000,
+      courses: [{ key: "AB1234", destination }],
+    },
+    root,
+    runner: {
+      node: process.execPath,
+      argumentsFor(command) {
+        return [command];
+      },
+      spawn(_command, argumentsFor) {
+        commands.push(argumentsFor[0]);
+        return completedChild({
+          code: 1,
+          stderr: `EACCES: permission denied, mkdir '${destination}'`,
+        });
+      },
+    },
+    wait: async (milliseconds) => waits.push(milliseconds),
+  });
+
+  assert.deepEqual(commands, ["sync"]);
+  assert.deepEqual(waits, []);
+  assert.equal(digest.verdict, "red");
+  assert.equal(
+    digest.message,
+    `Destination ${destination} is unreachable — permission denied at ${destination}; correct driveMountPath or destination permissions, then run: npm run watchdog`,
+  );
+  assert.doesNotMatch(digest.message, /crash|timeout/i);
+});
+
 function completedChild({ code, stdout = "", stderr = "" }) {
   const child = new EventEmitter();
   child.pid = nextFakePid++;
diff --git a/test/watchdog.test.mjs b/test/watchdog.test.mjs
index 2509712..4ec473d 100644
--- a/test/watchdog.test.mjs
+++ b/test/watchdog.test.mjs
@@ -219,6 +219,55 @@ test("refuses a run when the Drive mount is absent", () => {
   );
 });
 
+test("names an absent destination Drive root", () => {
+  assert.deepEqual(
+    watchdogVerdict({
+      sync: null,
+      verify: null,
+      preChecks: {
+        driveMount: { path: "/Volumes/Google Drive", present: true },
+        destinations: [
+          {
+            path: "/Volumes/Google Drive/My Drive/Modules/AB1234",
+            root: "/Volumes/Google Drive/My Drive",
+            present: false,
+          },
+        ],
+      },
+      attempts: 0,
+    }),
+    {
+      verdict: "red",
+      message:
+        "Destination /Volumes/Google Drive/My Drive/Modules/AB1234 is unreachable — expected Drive root /Volumes/Google Drive/My Drive; set driveMountPath and the destination to the mounted Drive, then run: npm run watchdog",
+    },
+  );
+});
+
+test("reports a destination permission refusal rather than a crash", () => {
+  assert.deepEqual(
+    watchdogVerdict({
+      sync: {
+        crashed: true,
+        report: null,
+        destinationPermission: {
+          code: "EACCES",
+          destination: "/Volumes/Google Drive/My Drive/Modules/AB1234",
+          path: "/Volumes/Google Drive/My Drive",
+        },
+      },
+      verify: null,
+      preChecks: {},
+      attempts: 1,
+    }),
+    {
+      verdict: "red",
+      message:
+        "Destination /Volumes/Google Drive/My Drive/Modules/AB1234 is unreachable — permission denied at /Volumes/Google Drive/My Drive; correct driveMountPath or destination permissions, then run: npm run watchdog",
+    },
+  );
+});
+
 test("reports an exhausted crash or timeout with its attempts and stderr tail", () => {
   assert.deepEqual(
     watchdogVerdict({