External Drive Is Read-Only on a Mac
Short answer
Check the format in Disk Utility or by pressing Command-I on the drive in Finder. If it says Windows NT File System or NTFS, macOS can read it but has no built-in write support — that is a licensing limitation, not a fault. Your options are reformatting to exFAT, writing to it from a Windows PC instead, or installing a third-party NTFS driver.
On this page
The drive mounts. You can open it, browse it, copy files off it. Try to copy anything onto it and the Finder refuses, or the paste option is greyed out entirely.
Nothing is broken. The drive is almost certainly formatted as NTFS, which is Microsoft’s filesystem, and macOS reads NTFS but does not write it. That is a deliberate limitation rather than a bug, and no amount of permission repair changes it.
Confirm the format first
Thirty seconds of checking saves you from fixing the wrong problem.
In Finder: select the drive, press Command-I for the info window, and read the Format line.
In Disk Utility (Applications → Utilities): select the volume in the sidebar and read the format shown beneath its name.
| Format shown | Can macOS write to it? |
|---|---|
| Windows NT File System (NTFS) | No, read-only |
| ExFAT | Yes |
| MS-DOS (FAT32) | Yes, with a 4GB file size limit |
| APFS | Yes |
| Mac OS Extended (Journaled) | Yes |
If it says NTFS, skip to the options below. If it says anything else and the drive is still read-only, the cause is something different and the last section covers it.
Option one: reformat to exFAT
The right answer for a drive you want to use with both a Mac and a PC, provided you can move its contents somewhere else first.
Reformatting erases everything on the drive. Copy the contents to your Mac or another drive and confirm the copy is complete and openable before going further. There is no way to convert a filesystem in place.
- Open Disk Utility and, from the View menu, choose to show all devices. This matters: you want the physical drive, not just the volume on it.
- Select the drive at the top of its group, not the indented volume.
- Click Erase.
- Set Format to ExFAT and Scheme to GUID Partition Map.
- Erase, then copy your files back.
exFAT is readable and writable by both macOS and Windows with nothing installed. Its weakness is the absence of journaling, which means an interrupted write can leave the filesystem inconsistent. Eject properly rather than pulling the cable, and it is fine.
For a drive that will only ever touch Macs, choose APFS instead. It is faster, handles snapshots, and is what macOS expects. For a mechanical Time Machine drive that must work with much older macOS versions, Mac OS Extended (Journaled) is still a defensible choice.
Option two: write to it from Windows
If the drive belongs to someone else, holds a Windows backup, or is simply too full to empty, leave it alone. Copy what you need from the Mac onto it using a Windows PC, or move files between the two machines another way.
For file-by-file transfers this is frequently less work than reformatting. A network share, a cloud folder, or simply emailing a document all avoid the problem entirely. Where the files are large, compressing a video before sending it is often the difference between a transfer that works and one that bounces.
Option three: install an NTFS driver
Third-party software adds NTFS write support to macOS. The established commercial options are widely used and work well enough that many people with mixed Mac and Windows setups simply buy one.
Two honest caveats. Anything writing to a filesystem at this level operates with deep system access, so install only well-known software from its developer’s own site. And these drivers often need updating after a major macOS release, during which the drive goes back to being read-only until a compatible version appears.
Free NTFS write solutions exist, including an undocumented method involving a system configuration file that circulates in forum posts. It is not supported by Apple, has a genuine record of causing filesystem corruption, and is not worth the risk on a drive holding data you care about.
When the format is fine and it is still read-only
If the drive is exFAT, APFS or Mac OS Extended and macOS still will not write to it, the cause is one of these.
A permissions flag on the volume. Select the drive in Finder, press Command-I, and expand the sharing and permissions section at the bottom. Tick the option to ignore ownership on this volume. That is the standard fix for a drive formatted on a different Mac or under a different user account.
A physical write-protect switch. SD cards and a few USB enclosures have one. Look at the hardware.
Filesystem errors. Disk Utility → select the volume → First Aid. macOS mounts a damaged volume read-only to prevent further damage, which is protective behaviour rather than a fault. If First Aid cannot repair it, copy the data off immediately and treat the drive as failing.
A Time Machine volume. macOS restricts direct writing to backup volumes by design. Nothing to fix.
An encrypted or BitLocker volume. A drive encrypted with BitLocker on Windows is not readable by macOS at all without third-party software, which presents differently but gets confused with the NTFS case.
Realistic expectations
For most people the answer is exFAT, and the whole exercise takes as long as copying the drive’s contents twice. A 1TB mechanical drive means an evening; an SSD means under an hour.
The honest limit is that there is no free, supported, safe way to write NTFS from macOS, and there has not been for many years. Apple has no reason to license it and Microsoft has no reason to make it easy. If you need a drive that both systems write to without software, exFAT is the format that exists for exactly that purpose.