Unable to record to External Hard drive

Hello -

I recently purchased the Pro Tools Express 10.3.4 for audio book recording. I can record to my computer (which I know is not ideal), but am unable to record directly to my external hard drive - Passport Ultra 1 TB -

When I try to connect them, I get an error message -5000 that reads I am not authorized to connect Pro Tools to the hard drive.

My computer is a MacBook Pro - with the Yosemite update.

Can someone help?

Many thanks