External hard drive read‑only on Mac? How to fix it

External drives · 7 min read · Updated June 2026

You plug in an external hard drive or USB stick and try to copy a file onto it — then macOS stops you cold with a message like "You don't have permission to modify the contents of this volume" or the drive simply shows up as read‑only in Finder. Frustrating, but fixable. There are four common causes, and each one has a clear solution. This guide walks through every cause in plain English, in the order you should check them.

Why your Mac can't write to your external drive

Before you try anything, it helps to understand that "read‑only" is not one problem — it is a symptom of several different problems. Treating the wrong cause wastes time and risks your data. Work through the list below in order; most people find their answer in the first two steps.

Cause 1: The drive is formatted as NTFS (a Windows format)

NTFS (short for New Technology File System) is the standard format Windows uses for hard drives. macOS can read NTFS drives just fine — you can open and copy files off them — but it cannot write to them without extra software. This is a deliberate limitation, not a bug.

How to check if your drive is NTFS

Open Finder, right‑click the drive in the sidebar, and choose Get Info (or press Command ⌘ + I).
Look at the Format line. If it says "Windows NT Filesystem" or "NTFS", that is your cause.
Alternatively, open Disk Utility (Applications → Utilities → Disk Utility), select the drive on the left, and read the format shown in the main panel.

How to fix an NTFS drive on Mac

You have two options:

Reformat to exFAT — exFAT is a format that both Mac and Windows can read and write to freely. It is the best choice if you share the drive between computers. Use Disk Utility: select the drive, click Erase, choose exFAT as the format, and confirm. Warning: erasing deletes everything on the drive. Back up your files first.
Reformat to APFS or Mac OS Extended — If you only ever use the drive with a Mac, these native formats give you better performance and full Time Machine support. APFS works best for solid‑state drives (SSDs); Mac OS Extended (also called HFS+) is fine for traditional spinning hard drives.
Use a third‑party NTFS driver — Apps like Paragon NTFS or Mounty add write support for NTFS to macOS without reformatting. This is useful when you cannot reformat (for example, a drive belonging to someone else). These are paid or freemium tools, not built into macOS.

Before you reformat any drive, copy everything off it to another location first. Erasing a drive in Disk Utility removes all files permanently and cannot be undone.

Cause 2: The drive is a dedicated Time Machine backup disk

Time Machine (Apple's built‑in backup system) can claim an entire drive for itself. When it does, macOS marks that volume in a way that prevents you — and other apps — from freely adding or deleting files on it. This protects your backups from accidental changes.

How to tell if Time Machine owns the drive

Open System Settings → General → Time Machine (or System Preferences → Time Machine on older macOS). If your drive appears there as a backup destination, Time Machine has claimed it.
The drive may also show a special icon in Finder that looks different from a regular external drive.

How to fix it

Use a separate drive for general file storage. Keeping Time Machine backups on their own dedicated disk is good practice anyway — it means your backups are always separate from your files.
Repurpose the drive — Remove it from Time Machine in System Settings, then reformat it in Disk Utility. This erases all the backups stored on it, so make sure you have another backup strategy in place first.
Partition the drive — Disk Utility lets you split one drive into two separate partitions. You can give Time Machine one partition and keep the other for general use. (Note: both partitions share the same physical disk, so a hardware failure affects both.)

Cause 3: The drive is full

This one sounds obvious, but it is easy to miss. When a drive has zero free space, the operating system has nowhere to write new data — so it effectively acts read‑only. macOS does not always show a helpful error message explaining this.

How to check free space

In Finder, select the drive in the sidebar and press Command ⌘ + I to open Get Info. Look for the Available line.
You can also hover over the drive icon in Finder — the status bar at the bottom of the window (enable it via View → Show Status Bar) shows available space.

How to fix it

Delete files you no longer need from the external drive, or move some of them to another location to free up room. Once there is space available, writing should work again.

Cause 4: Permissions or ownership settings

Every file and folder on a Mac has ownership metadata — a record of which user account "owns" it. When a drive was formatted or used on a different Mac (or a different user account), those ownership records can conflict with your current account, locking you out even though you are sitting right in front of the machine.

How to fix permissions with "Ignore ownership on this volume"

In Finder, click the external drive in the sidebar to select it.
Go to File → Get Info (or press Command ⌘ + I).
Scroll to the bottom of the Get Info window. You will see a Sharing & Permissions section.
Click the padlock icon at the bottom‑right and enter your Mac password to unlock changes.
Tick the checkbox labelled "Ignore ownership on this volume."
Close the window. Your Mac will now treat all files on that drive as belonging to you, regardless of what the stored ownership records say.

This change is safe and reversible — you can untick it the same way. It does not alter any files on the drive itself; it only changes how your Mac interprets the drive's ownership data.

Cause 5: Filesystem errors (run First Aid)

Sometimes a drive that was not ejected properly — because of a power cut, a cable being pulled, or a crash — can develop minor filesystem errors. macOS may respond by mounting the drive as read‑only to protect it from further damage.

How to run First Aid

Open Disk Utility (Applications → Utilities → Disk Utility).
Select the external drive or its volume in the left‑hand list.
Click First Aid in the toolbar, then click Run.
Disk Utility will scan and attempt to repair any errors it finds. When it finishes, eject and reconnect the drive, then try writing to it again.

If First Aid reports errors it cannot fix, the drive may have physical damage. In that case, copy your data off it as quickly as possible and replace the drive.

Step‑by‑step diagnosis: the right order to check

Check the format — Open Disk Utility or Get Info and look at the format. If it says NTFS, that is almost certainly the cause.
Check free space — Confirm the drive has room to accept new files.
Run First Aid — Let Disk Utility repair any filesystem errors.
Toggle "Ignore ownership on this volume" — Try this in the drive's Get Info window.
Reformat as a last resort — If nothing else works and you have backed up your data, erasing and reformatting the drive usually solves the problem completely.

Choosing the right format: exFAT vs APFS vs Mac OS Extended

When you do reformat, pick the right format for your use case:

exFAT — Best for drives shared between Mac and Windows. Both operating systems can read and write to it without extra software. Good for USB sticks and portable drives you carry between computers.
APFS (Apple File System) — Apple's modern format. Best for Mac‑only SSDs. Supports encryption, snapshots, and efficient storage. Required for newer Macs' internal drives.
Mac OS Extended (HFS+) — Apple's older format. Works well for Mac‑only spinning hard drives and is the traditional choice for Time Machine backup disks on older versions of macOS.

If you need to move a Photos library to it, choose APFS or Mac OS Extended — exFAT does not support all the metadata that Photos relies on as efficiently.

A note on your Mac's internal storage

If a temperamental external drive has become your main overflow for files, it is worth freeing up space on the Mac itself so you are not entirely dependent on it. Storage Bee shows you what is taking up room on your Mac's internal disk — large files, old downloads, Trash that hasn't been emptied — so you can make informed decisions about what to keep and what to remove. It is a small precaution that pays off if your external drive ever causes trouble again.

See what's eating your Mac's internal storage

Storage Bee gives you a clear view of large files, old downloads, and forgotten Trash — so you can free space safely, without guesswork.

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Frequently asked questions

Why is my external hard drive read‑only on Mac?

The most common reason is that the drive is formatted as NTFS — a Windows format that macOS can read but not write to. Other causes include the drive being a dedicated Time Machine disk, the drive being completely full, or file permission settings that restrict access. Running Disk Utility's First Aid can also reveal filesystem errors that trigger read‑only mode.

How do I fix NTFS read‑only on Mac without third‑party software?

The built‑in fix is to back up all your data, then reformat the drive in Disk Utility to exFAT (for Mac and Windows compatibility) or APFS / Mac OS Extended (for Mac‑only use). Reformatting erases everything on the drive, so backing up first is essential. If you need to keep NTFS, you will need a third‑party NTFS driver app.

What does "Ignore ownership on this volume" do on Mac?

It tells macOS to stop enforcing the file ownership records stored on that external drive. Those records are written for a specific user account — often on a different Mac — so they can accidentally lock you out on your own machine. Ticking "Ignore ownership on this volume" in the drive's Get Info window makes macOS treat all files as yours and often restores full write access instantly.

Should I format my external drive as exFAT or Mac OS Extended?

Choose exFAT if you need the drive to work on both Mac and Windows computers — exFAT is compatible with both without extra software. Choose Mac OS Extended (also called HFS+) or APFS if the drive is Mac‑only; these formats support Time Machine and macOS‑specific features. APFS is best for SSDs; Mac OS Extended is fine for spinning hard drives.

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