If you don’t have permission to use files on a 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 are 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.
Select the disk, then choose File > Get Info.
Click the disclosure triangle next to Sharing & Permissions to expand that section.
If necessary, click the lock icon , then enter an administrator name and password.
Select the “Ignore ownership on this volume” checkbox.