Need help using Windows 7 Backup

[greenwizard88]

So I installed a 2nd HDD in my PC to use as a backup drive. I have a 640gb hard drive partitioned into 540gb and 100gb, and the 2nd HDD is partitioned the same way. I want Windows backup to back up the 540gb partition onto the other 540gb partition. Whenever I do this, it backs up about 154gb of 276gb, and then tells me that it's out of space. Why is it doing this, and how do I fix it?

Thanks Neowinians :)