How do I change from Read only to Read/Write on MyBook Duo 8TB Thunderbolt?

When I try to check Read/Write speeds on my new MyBook Duo, it give me an error message “Speed test can not be run on selected location because it is Read only”.  

How do I change it to Read/Write?  

I would like to know what kind of read/write speeds I am getting with this external hard drive.  

I have an iMac and have it as RAID1.   I am using it primarily for all my Photography files for Lightroom and Photoshop; as well as backup, timemachine, store movies, etc.  

Hi gemsPhtgrphy, welcome to the WD Community. Out of the box the My Book Duo comes formatted for Windows computers using NTFS format. If you haven’t formatted the My Book Duo for Mac you won’t be able to write any information. You can see more information on this on page 81 of the manual.