Q The 500 GB hard disk in my newish iMac seems to be running out of free space remarkably quickly. About This Mac suggests that I have more music and movies than I thought, and I only have 104 GB left. How come?
A It is easy to become alarmed as disk space becomes used, but you still have ample free disk space to ensure that OS X does not become slow and turgid.
Use the information provided by About This Mac to guide a determined housekeeping session, rather than risk throwing the baby out with the bathwater, as could happen if you let Disk Diag loose to trash files. Check whether some of your media libraries might be getting doubled up: presuming that you are now using Photos, check to see whether you still have old iPhoto or Aperture libraries in your Pictures folder.
You may find that iTunes tracks are kept in the iTunes Music folder (traditional) as well as iTunes Media/Music (more recent). Use iTunes to identify duplicate tracks for weeding. Then work carefully through items in your Documents folder, to see which can be safely archived. You may even find some large movie files lurking in both Documents and Movies, perhaps.
There is a rule of thumb that we use 20% of our document files 80% of the time, and the remaining 80% of the documents only 20% of the time. This is based on the Pareto Principle. If you are still concerned that you are running out of space, try to identify the largest files and folders among the 80% which are least used, and move them off your hard disk to archive.
If you find your disk short of space after all that, consider getting it upgraded to 1 TB or more, or purchasing an external drive on which to store those 80%.
Updated from the original, which was first published in MacUser volume 29 issue 13, 2013.