Cannot choose disk SU when creating a SLP

Windows Server 2008 R2, NBU 7.6.0.1, License Trial Full.

I create a 4 SUs : 3 SU is disk - basicdisk, 1 SU is MediaManager - tape.

When I create a SLP: In step : New Storage Operation > Properties. Operation = Backup. In Destination Storage Attributes>Destination drop box

=> It only show SU from MediaManager - tape while other SU didn't show.

I read documents about SLP and find there is no requirement for SU in SLP. 

Is it something problem or I am missing some configuration steps?