I would be glad to help. There are a number of reasons you could be having this problem. To better understand the problem can you make sure all of your drives are plugged in and connected, and then open the Disk Utility app from the Launchpad icon on your Dock. The Disk Utility may be inside a folder labeled Tools or Utilities. Once you have the app open, let me know the size of all of your drives, and how much free space each one has.