Your other drives are working, you're just not using them! Move all your music, photos, videos, documents to the other unused drives.
Assuming you have a Windows machine we can tell you how to automatically send your libraries to a different drive to clear up the C: drive if you tell us the OS you are using - XP, Windows 7, etc
I had a Windows 8.1 machine and was shocked at how much space the system restore was using. I was able to delete all but the latest restore point and regained a LOT of hard drive space. Then I adjusted the settings so that it couldn't happen again.