WD SSD external drive not authorized

[Geoff_Coupe]

You may need to create a folder first to set up the required permissions. See:

Cannot backup Roon to attached Time Machine WD Passport [Resolved]

Where is your core? Is it on the same iMac or on another computer? If it’s on another computer then you need to map to the iMac drive share via network. The core is controlled by any computer but the drive locations are with respect to the core not the control application.