If you can’t copy and paste files from Mac to an external hard drive, most likely that your external hard drive doesn’t have the Read & Write permissions. To give permission, do the following:
In Finder, right-click on your external drive → select Get Info.
In the Sharing & Permissions section, check the Permission.
Click the Lock icon.
Enter your username and your password.
Select Read & Write for the privilege type.
Click the Lock icon again and try to copy files to your external drive.