After connecting an external hard drive to a Mac, you expect to have full read-write access to this drive to view the files and modify them. However, sometimes you may find your external hard drive read-only, especially when you are using an unfamiliar Mac or unfamiliar external hard drive.
The post lists three most possible reasons and their solutions.
By default, Microsoft NTFS file system is not fully compatible with Mac OS. Files on NTFS-formatted drives will appear read-only or locked on Mac. You can check the file system by right-clicking the icon of the external HD and select Get Info. The file system information will be shown beside Format item.
If NTFS file system is confirmed, the best and easiest way to bridge this gap is to use an NTFS driver for Mac. The NTFS driver for Mac can enable you to gain full read and write permission to the drive easily.
Another reason why you can’t write to an external hard drive on Mac could be your lack of write permission. The administrator of this Mac can set up different permissions to every single volume or disk, so you may don’t have the permission to write to the external hard drive on this Mac.
To change the read-only permission, you need to：
Some minor formatting errors or logical errors on the external hard drive may result in the disk in a read-only state. Those errors may not make the hard disk totally unrecognized, unreadable or corrupted but do make your drive lock you out. You can repair the errors using First Aid or give your drive a complete format or do them both.
To repair the external hard disk:
To format the external hard disk:
The issue that an external hard drive shows read-only in Mac OS is solvable. The reasons why you are exclusive from the write access to the drive can be incompatible file system, read-only volume permission and corruption. Find the related solution and read-write to your external hard drive freely.