This is what I did anyhow…

Here my external disk is Sling and it has the proper permissions root:admin. Before it had root:wheel which wasn’t allowing me to write to the disk as I did before.

To get the right permissions I used sudo chown root:admin Sling/. I thought that this permission would only apply one time, but I ejected and re-mounted and it kept the permission.

Hopefully this helps.