Hello, glad to help you, here are my suggestions, you can follow:
(1) Check the file system in Disk Utility. If its file system is Windows NTFS, you need to use some tools to enable Mac to write it, because Mac can't write an NTFS-formatted drive. iBoysoft NTFS for Mac is a good choice to try, which can provide seamless read-and-write functionality for the NTFS drives on Mac.
(2)Change the permissions. In the Sharing & Permission section, change the Read only to Read & Write, click the yellow padlock then enter your password.
(3)Enable Ignore ownership on this volume.
(4)Check the eternal hard drive's free space. If there isn't enough space, delete some unimportant files.
(5)Repair it with Disk Utility First Aid.
(6)Format the drive on Mac. Pay attention to backup your data before this process.
Hope you can use your drive normally as before.
Also read: Can't Write to External Hard Drive on Mac
