Backing up from My Book to another Mybook

I use a WD 2TB External Drive as my primary hard drive for storing photos through Aperture. I want to back up the hard drive onto another 2TB WD external hard drive.

I have tried dragging and drop but it keeps coming up with an error.

Can anyone assist with how to do this.

Regards

Hello,

What is the error you get when you try to drag and drop your files from one My Book to another?