MacOS – Does a bootable OS X drive need more free space than the installer

According to this About.com page written by Tom Nelson:

I would say keep at least 15% of your startup drive free at all times...so that some basic OS X maintenance scripts will have sufficient free drive space to run. This includes OS X's built-in disk defragmentation system, memory swap space, and enough space to create cache and temp files when OS X starts up, while still leaving room for basic applications, such as email and web browsers, to use free space as needed.

For others with similar problems, you don't need a special utility to find out where all your GB have gone on a Mac.

First, turn on invisible files. On newer Macs, you can do that with Command Shift Period. On older Macs, you do need a quick defaults terminal command.

Once you've done that, open your hard drive, and put it in to List View

Then go to the View menu and choose Show View Options Check Calculate all Sizes

Now wait. It can take a while because there are lots of sizes to add up, but eventually it will show you the sizes of every folder. You can use the disclosure triangles to open more and more folders and drill down to where all of your data is being used. I also usually sorts by the Size column so the big stuff is right at the top.

As a professional Mac consultant, this is the method I use to find people's data, I don't bother with any 3rd party utilities for this.