Fix Read-Only External Drive on Mac: Step-by-Step

If your external drive suddenly shows as read-only on your Mac, you can’t copy, delete, or edit files on it. This guide walks you through the most common causes and fixes, from checking the physical lock switch to repairing the drive with Disk Utility and adjusting permissions. No technical background is needed—just follow the steps in order.

What You’ll Need

• The external drive (USB, Thunderbolt, or SD card) that is read-only
• Your Mac’s administrator password (for some steps)
• A second external drive or cloud storage to back up important files (recommended before any repair)
• Optional: a USB hub or different cable to test connectivity

Step-by-Step

1. Check the physical lock switch. Many external drives and SD card adapters have a tiny sliding switch on the side or bottom. If it’s set to the “lock” position, the drive becomes read-only. Slide it to the unlock position, eject the drive, and reconnect it.
2. Eject and reconnect the drive. Click the eject icon next to the drive in Finder, unplug it, wait a few seconds, and plug it back in. Sometimes a simple remount clears the read-only state.
3. Verify the drive’s format. Open Disk Utility (in Applications > Utilities). Select the external drive in the sidebar and look at the format listed under the drive name. If it says “MS-DOS (FAT)” or “ExFAT,” the drive may be formatted for Windows and not fully compatible with macOS write access. If it says “Mac OS Extended” or “APFS,” proceed to the next step.
4. Run First Aid. In Disk Utility, select the drive (the top-level item, not the volume below it) and click First Aid. Click Run and wait for it to finish. This checks for directory errors that can cause read-only behavior. If errors are found, First Aid will attempt to fix them.
5. Check and repair permissions (for Mac-formatted drives). If the drive is Mac OS Extended or APFS, select the volume (the indented item under the drive) in Disk Utility, then click First Aid and run it. This repairs permissions on the volume. After it finishes, eject and reconnect the drive.
6. Reset the drive’s mount options. Open Terminal (in Applications > Utilities) and type the following command, then press Return:
   sudo diskutil mountDisk /dev/disk2
   Replace disk2 with your drive’s identifier (you can find it in Disk Utility’s info panel or by typing diskutil list in Terminal). This forces the drive to remount, sometimes clearing read-only flags.
7. Check the drive’s mount permissions in Finder. Select the drive in Finder, press Command+I to get Info, and expand the Sharing & Permissions section. Ensure your username has Read & Write access. If it says Read only, click the lock icon, enter your password, and change the privilege to Read & Write.
8. If the drive is NTFS-formatted, install a driver or reformat. macOS can read NTFS drives but cannot write to them by default. You have two options: either reformat the drive to a Mac-compatible format (which erases all data) or install a third-party NTFS driver (like Paragon or Tuxera) that enables write access. Since this guide avoids brand endorsements, search for “NTFS driver for Mac” and choose a reputable option. Reformatting is the built-in solution but requires backing up your data first.
9. Try a different USB port or cable. Faulty connections can sometimes cause the drive to mount read-only. Test with another cable and port, preferably one directly on the Mac rather than through a hub.
10. Restart your Mac. If nothing else works, save any open documents and restart your Mac. This clears system caches and can resolve temporary glitches.

Common Mistakes

• Skipping the physical lock check. The lock switch is the most common cause, yet it’s often overlooked. Always check it before doing anything else.
• Running First Aid on the wrong item. You must select the volume (the indented item) for permission repairs, and the whole drive for directory repairs. Running it on only the top-level drive may not fix volume-level issues.
• Assuming all drives are Mac-formatted. If the drive was used with Windows, it’s likely NTFS or ExFAT. NTFS is read-only by default on macOS, so no amount of permission fixing will help.
• Reformatting without a backup. Reformatting erases everything on the drive. If you haven’t backed up your files, you’ll lose them permanently.
• Using Terminal commands without knowing the disk identifier. Typing the wrong disk number can affect the wrong drive. Always double-check with diskutil list first.

Tips

• Keep your drive formatted as ExFAT if you need to use it on both Mac and Windows. ExFAT is read-write on both systems without extra drivers.
• Use Disk Utility’s First Aid regularly to prevent directory errors that can lead to read-only states.
• Always eject the drive properly before unplugging. This reduces the risk of file system corruption.
• If you use Time Machine, note that Time Machine volumes are intentionally read-only to the user to protect backups. That’s normal behavior, not a problem.
• Consider using a powered USB hub for drives that need more power than your Mac’s port provides. Insufficient power can cause erratic behavior, including read-only mounting.

Troubleshooting

If the drive is still read-only after following all steps, try these additional checks:

• Check System Information. Go to About This Mac > System Report > USB (or Thunderbolt). Find your drive and see if it reports any errors. If it doesn’t appear, it’s a connection issue.
• Test the drive on another computer. If it works as read-write on a Windows PC, the problem is macOS-specific. If it’s read-only there too, the drive’s file system or hardware is likely the issue.
• Try a different user account. Create a temporary new user in System Settings > Users & Groups, log in as that user, and see if the drive is writable. If it is, your original account has a permissions issue that can be fixed by resetting the home folder permissions.
• Check the drive’s S.M.A.R.T. status. Some drives have built-in diagnostics. You can check this in Disk Utility (if supported) or via third-party tools. A failing drive may become read-only to protect your data.
• If the drive is encrypted with FileVault, ensure it’s unlocked and the password is correct. An encrypted drive that fails to unlock properly may mount read-only.

Frequently Asked Questions

Why is my external drive read-only on Mac but not on Windows?
Most likely the drive is formatted as NTFS. macOS can read NTFS but cannot write to it without third-party software. Reformatting to ExFAT or installing an NTFS driver will fix it.

Can I fix a read-only drive without losing data?
Yes, in most cases. The steps above (lock switch, First Aid, permissions) do not erase data. Only reformatting erases data, and that should be a last resort after backing up.

What does “read-only” mean exactly?
It means you can open and view files on the drive, but you cannot create new files, delete existing ones, or modify them. The drive’s file system is mounted with write access disabled.

How do I reformat an external drive on Mac?
Open Disk Utility, select the drive, click Erase, choose a format (APFS or Mac OS Extended for Mac-only, ExFAT for cross-platform), and click Erase. This deletes all data, so back up first.

Is it safe to use Terminal commands to fix a read-only drive?
Yes, if you use the correct disk identifier and command. The diskutil command is safe for unmounting and remounting drives. Avoid commands like dd or mkfs unless you know exactly what they do.

Why does my SD card become read-only after a while?
SD cards have a physical lock switch and also a controller that can set the card to read-only if it detects errors or aging. Try the lock switch first; if that’s not it, the card may be failing, and you should back up immediately.

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