Hard Disk free space dissapears and reappears

Question

Hi There,
Lately I've been noticing some very strange fluctuations in the reported "free space" on my HD. I let the disk get quite full (~1GB free or so) and then starting backing up and transferring data to my external HD. But when I started deleting these items the free space on the Hard Disk didn't increase. Then, one week later, it spontaneously goes from 2GB free to 13GB free (which I assume is the right ballpark).
Any thoughts as to what might be going on?

Answer 1

Your diagnostic priority, in order, is Disk Utility (Repair Permissions and Repair Disk), then Disk Warrior (a much more powerful version of the Repair Disk function in Disk Utility).
Mac Janitor is not really going to help much here. Many Mac users turn to Mac Janitor in a knee-jerk way, like the old "rebuilding the desktop" on OS 9, but if your problem is loss of gigabytes of disk space, there is simply no way the plain text logs maintained by MacJanitor can expand to gigabytes in size unless they are writing to the logs constantly without a break.
Part of your free space fluctuation may be explained by the automatic virtual memory swap file management does, as VM swap files come and go. The less RAM you have, especially under 1GB, the more likely you have VM swap files building up. When you have more than 10GB disk space free, the swap file movements aren't too noticeable. When you get down below 5GB free, swap files take up proportionally more of your free space and have a much higher chance of destabilizing the machine.