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 select an option to ignore ownership of those files. When this option is selected, your Mac treats all files on the disk as though they are owned by the current user, no matter who actually owns them.
On your Mac, click the Finder icon in the Dock to open a Finder window.
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 “Ignore ownership on this volume” tickbox.
Note: If you have used the external drive for Time Machine backups, this option doesn’t appear.