Users of older versions of Windows know that sometimes disk space reporting can go awry. OS X is not immune to this oddity, though it can take more than a disk scan to fix the problem.
If you've recently updated to OS X Yosemite and noticed that the operating system is reporting available space in the petabyte range and you're reasonably confident you don't have any petabyte-sized hard drives, you might need to give Time Machine a talking to.
As Simple Help's Ross McKillop explains, the OS pumps out hidden backups that can sometimes confuse the issue. Unfortunately, you'll need to roll your sleeves up to get it behaving again. Put in straightforward terms, a root folder labelled ".MobileBackups" was the source of the problem.
Depending on your configuration, you should be able to delete the contents of this folder to not only reclaim drive space, but get a proper reading of available space. Hit up Simple Help for a step-by-step guide.
How to Fix a Mac with "PB" Disk Space [Simple Help]