If you don’t have permission to use files on a Mac disk

If you don’t have permission to use the files on a disk that’s connected to your Mac and you are not the administrator of the Mac, ask the administrator to give you access to the disk.

If you’re the administrator, you can change the read and write privileges for those files.

On your Mac, select the disk, then choose File > Get Info.
Click the arrow next to Sharing & Permissions to expand that section.
If the lock at the bottom right is locked , click it to unlock the permissions.
Select the type of user you want to give access to, then choose “Read only” or Read & Write from the Privileges menu.

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macOS User Guide

If you don’t have permission to use files on a Mac disk