B2D and Duplication error

hi ,exec's boys and girls
 
i have a question about B2D and Duplication
 
i backup my exchange server (220 Go -Full and Diff) and a file server (140 Go Full and Diff) ,my backup
distination is a B2D folder (250 Go) then i duplicate to tape Library
My problem is that my diff backup dosen't succeed because full backup are duplicated
to tape and then erased by diff or the other full backup. (opp = 0 because the low size of my B2D)!!!!!!
i need the B2D for GRT but when using it ;diff backup cannot be succed ;
How do i proceed to duplicate job and a GRT restore ???