What is the concept behind swapping of drives while backup?

Hi All,

Why is there swapping of drives while backup is in use? Let myself be clear.

  then --------------------->      then --------------->   

Why not fixed drives are being used for the same backup? What is the concept behind it?

Now the issue that I was stuck in is -

With time the drives status was of image 3 for the same backup job.

We have configured our environment such that for some specific backup policy only these drives 1,3 & 5 should be used and for other policy drives 2,4 & 6 should be used. 

Now while a backup was running I suddenly needed to run a critical backup which according to the policy will use the drives 1,3 & 5 (odd numbers).

But we can see that the drives are already being used in image 3.

So my question is -

Is there anyway to manually swap the drives between running backup (as we can see that this is being done automatically)? Because scenario that I was stuck in may occur to anyone.

Thanks.