Norton Security Suite will not recognize my external drive for backup

I am running Windows 7 ProPremium 64-bit and both Norton Security Suite v4.0.0.127 (free for comcast customers)  will not recognize my external hard drive as a destination location for my backup.   I am using a Seagate GoFlex and I have mounted the drives as X,Y and Z.

 

Can anybody help me on this?