Hard drive always busy at 100%?

I just installed a new SSD to my computer (It had already one SSD and one HDD). I also swapped some SATA cables to get better transfer rates on the SSDs (They were on SATA 3Gb/s while I had two 6Gb/s free at another place). I rebooted Windows, everything seemed to work fine.

Since then (It was yesterday), my HDD is very often totally busy (100% in the Task Manager).

The thing is... I don't have anything installed on the HDD. All my apps are on the first SSD, and the only thing I put on the second one is BF4 and Origin. I didn't change anything else than what I said. Is there any way to see what's happening here ? Or to see what software is using my HDD ? Because it's really slowing Windows down (I feel like, the Cortana search is often very slow, or unresponsive, and that didn't happen before).