How do i move the device backup folders

Hi there, I now have my iTunes folder on an ext drive, which is great news. The bad news is that the device backup files are still on my HD taking up loads of space as i have now the 128GB iPad mini. is there another way i can moves these files as well, so when i back up the ipad it will back up to the EXT drive on the the local HD ?

I have iTunes 11.1.3

Thanx in advanv