Recently I have face issue while adding the Buffalo NAS Storage with the backup exec 15.

Error :- 

SSL Certificate has been expired , Unable to create trust with Storage.

Cause :- 

Some NAS storage provide Preinstalled backup exec agent and that certificated was expired

Solution :- 

Remove the backup exec agent from NAS storage appliance and try to configure again,

Now I am able configure as backup storage without any error !