diff --git a/projects/core-api/src/main/java/dan200/computercraft/api/filesystem/IWritableMount.java b/projects/core-api/src/main/java/dan200/computercraft/api/filesystem/IWritableMount.java
index 7fb0d0538..1bebaf3e8 100644
--- a/projects/core-api/src/main/java/dan200/computercraft/api/filesystem/IWritableMount.java
+++ b/projects/core-api/src/main/java/dan200/computercraft/api/filesystem/IWritableMount.java
@@ -78,4 +78,15 @@ public interface IWritableMount extends IMount {
     default OptionalLong getCapacity() {
         return OptionalLong.empty();
     }
+
+    /**
+     * Returns whether a file with a given path is read-only or not.
+     *
+     * @param path A file path in normalised format, relative to the mount location. ie: "programs/myprograms".
+     * @return If the file exists and is read-only.
+     * @throws IOException If an error occurs when checking whether the file is read-only.
+     */
+    default boolean isReadOnly(String path) throws IOException {
+        return false;
+    }
 }
diff --git a/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java b/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java
index 371c77f1b..c5d393ad7 100644
--- a/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java
+++ b/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileMount.java
@@ -143,6 +143,16 @@ public class FileMount implements IWritableMount {
         return file.exists() && file.isDirectory();
     }
 
+    @Override
+    public boolean isReadOnly(String path) throws IOException {
+        var file = getRealPath(path);
+        while (true) {
+            if (file.exists()) return !file.canWrite();
+            if (file.equals(rootPath)) return false;
+            file = file.getParentFile();
+        }
+    }
+
     @Override
     public void list(String path, List<String> contents) throws IOException {
         if (!created()) {
diff --git a/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapperMount.java b/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapperMount.java
index 7e8f7c60c..0ec038aa9 100644
--- a/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapperMount.java
+++ b/projects/core/src/main/java/dan200/computercraft/core/filesystem/FileSystemWrapperMount.java
@@ -94,6 +94,15 @@ public class FileSystemWrapperMount implements IFileSystem {
         }
     }
 
+    @Override
+    public boolean isReadOnly(String path) throws IOException {
+        try {
+            return filesystem.isReadOnly(path);
+        } catch (FileSystemException e) {
+            throw new IOException(e.getMessage());
+        }
+    }
+
     @Override
     public void list(String path, List<String> contents) throws IOException {
         try {
diff --git a/projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java b/projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java
index aa7b1c899..ed9a818ed 100644
--- a/projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java
+++ b/projects/core/src/main/java/dan200/computercraft/core/filesystem/MountWrapper.java
@@ -61,8 +61,12 @@ class MountWrapper {
         return writableMount == null ? OptionalLong.empty() : writableMount.getCapacity();
     }
 
-    public boolean isReadOnly(String path) {
-        return writableMount == null;
+    public boolean isReadOnly(String path) throws FileSystemException {
+        try {
+            return writableMount == null || writableMount.isReadOnly(path);
+        } catch (IOException e) {
+            throw localExceptionOf(path, e);
+        }
     }
 
     public boolean exists(String path) throws FileSystemException {
diff --git a/projects/core/src/test/java/dan200/computercraft/core/filesystem/FileMountTest.java b/projects/core/src/test/java/dan200/computercraft/core/filesystem/FileMountTest.java
new file mode 100644
index 000000000..7e2a03093
--- /dev/null
+++ b/projects/core/src/test/java/dan200/computercraft/core/filesystem/FileMountTest.java
@@ -0,0 +1,75 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2022. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+package dan200.computercraft.core.filesystem;
+
+import com.google.common.io.MoreFiles;
+import com.google.common.io.RecursiveDeleteOption;
+import dan200.computercraft.api.filesystem.IWritableMount;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Assumptions;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.attribute.PosixFileAttributeView;
+import java.nio.file.attribute.PosixFilePermission;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class FileMountTest {
+    private static final long CAPACITY = 1_000_000;
+    private final List<Path> cleanup = new ArrayList<>();
+
+    @AfterEach
+    public void cleanup() throws IOException {
+        for (var mount : cleanup) MoreFiles.deleteRecursively(mount, RecursiveDeleteOption.ALLOW_INSECURE);
+    }
+
+    private Path createRoot() throws IOException {
+        var path = Files.createTempDirectory("cctweaked-test");
+        cleanup.add(path);
+        return path;
+    }
+
+    private IWritableMount getExisting(long capacity) throws IOException {
+        return new FileMount(createRoot().toFile(), capacity);
+    }
+
+    private IWritableMount getNotExisting(long capacity) throws IOException {
+        return new FileMount(createRoot().resolve("mount").toFile(), capacity);
+    }
+
+    @Test
+    public void testRootWritable() throws IOException {
+        assertFalse(getExisting(CAPACITY).isReadOnly("/"));
+        assertFalse(getNotExisting(CAPACITY).isReadOnly("/"));
+    }
+
+    @Test
+    public void testMissingDirWritable() throws IOException {
+        assertFalse(getExisting(CAPACITY).isReadOnly("/foo/bar/baz/qux"));
+    }
+
+    @Test
+    public void testDirReadOnly() throws IOException {
+        var root = createRoot();
+        var mount = new FileMount(root.toFile(), CAPACITY);
+        mount.makeDirectory("read-only");
+
+        var attributes = Files.getFileAttributeView(root.resolve("read-only"), PosixFileAttributeView.class);
+        Assumptions.assumeTrue(attributes != null, "POSIX attributes are not available.");
+
+        assertFalse(mount.isReadOnly("read-only"), "Directory should not be read-only yet");
+        attributes.setPermissions(Set.of(PosixFilePermission.OWNER_READ, PosixFilePermission.OWNER_EXECUTE));
+        assertTrue(mount.isReadOnly("read-only"), "Directory should not be read-only yet");
+        assertTrue(mount.isReadOnly("read-only/child"), "Child should be read-only");
+    }
+}