Backing up from My Book to another Mybook

[Tigers]

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

[Hamlet]

Hello,

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