Storage unit issue

Hi Everyone 

I am very much beginner to Netbackup . We just  installed 5250 in our environment . I created a Storage unit and took a full backup successfully. It was on Basic Disk . After that i created a new schedule in that same policy in which I configured to take incremental backup after 2 days . But my incremental backup got failed and I  got the error that storage unit full . I have 9TB of unallocated space . Kindly some one  tell me how to configure a very basic running setup for it ? How I can use this 9 TB unallocated space ? I have 3 servers whom I want to take full backup after every 1 week and I want to take incremental backup of them after every two days with both of their retention level set to 3 . My understanding was that  i simply need to create three storage units and Tie them with three separate policies . Each policy will contain two schedules ( one for full backup and second for incremental backup ). I am assuming that These three storage units will automatically claim unallocated space if they run out of space but looks like I am wrong. I tried to read documentation but could not understand and I cant get a proper training on this so please help me in design this simple solution with full steps . Thanks :)