Right-click your hard disk and select 'Get Info', if you find the external hard drive in NTFS format, that's what the cause is.
Why can you only read the external hard drive that is connected to your Mac? There are three possible reasons. Why Is Your External Hard Drive Read-Only on Mac When your external hard drive or other external storage devices show up as 'Read Only' on your Mac, you can: Right-click your external storage device, select 'Get Info', and if you see 'You can only read' at the bottom, you are encountering one of the commonest problems with an external hard drive. If you have any clue to fix this problem, please tell me. I could only read and view saved data on the drive, but can't use the data on it. ' Hi there, do you know how to make the external hard drive accessible again when it shows as 'Read Only' on Mac? I'm having this problem that my Seagate hard drive suddenly turned to read-only this morning when I connected it to Mac. The External Hard Drive Shows Up as 'Read Only' on Mac, Help! Open Disk Utility > Select the external hard drive and click 'First Aid' > Click 'Run'. Right-click external drive > 'Get Info' > 'Sharing & Permissions' > Check 'Ignore ownership on.'. Open 'Applications' > 'Utilities' > Launch 'Disk Utility' to 'Erase' external drive > Restore formatted data.
