Hello and Welcome to TSF!!!
I must ask you why do you need that huge partition on which there will be operating system and applications???
You could lose data again mate if you extend C: partition with your spare HDD...
I think it is better to leave it this way. You have one HDD,
C: partition and you use it to install operating system and applications on it.
Another HDD, D: partition you can use for data store such as Movies, images, music etc.
I cannot understand even why you have 500GB system partition.
I would divide that also on two or maybe three smaller partitions
[lets say C: to be 100GB and two partitions to be 200GB each] and have one big 500GB partition for backup :)
Anyway, if you really want to increase size of C: partition,
you can use software like Acronis Partition manager
or Partition Logic
Partition Logic Download Link
Hope this will help you mate
Keep us posted...