Peace Freak! wrote:
I just bought a 3 TB SATA 3 hard disk to use in an external case for backing up my computer and other files. I put it in a case and launched Disk Utility to find that it only shows up as having 800 GB available.
This may not be the whole answer to your question, but this Web page
discusses "high capacity" disks, those above 2.2TB. The thing that makes me think that it's related to your problem is that 800 GB is exactly 3 TB less 2.2 TB.
Did you partition that drive? If not, what is the partition type? That Web page suggests that a GUID partition map may be what you need.