Missing 20GB of hard drive space
Hello all .
I seem to be missing significant hard drive space , almost exactly 20GB worth.
I have a 750GB internal drive on my Macbook.
When I open finder , and look at the amount of space used on my internal drive "Macintosh HD",
[either by looking in the "size" column , or by opening a "get info" window on the root folder for the drive]
it currently says "716.04 GB used"
However , when I open this root folder , and do the addition of the sizes of the folders + files there [either on paper , or by selecting all the folders and files and doing a collective "Get info"] , the total comes to "695.8 GB"
Even when I un-hide all hidden files and folders [either by using the third party app "Houdini" or by the "chflags" command in terminal , it still says the total of files and folders within my root hard drive folder are 695.8 GB. ...
......but again , examining the root drive , I have apparently used 716.04 GB .
[My library folder has also been unhidden permanently long ago]
What is using this extra 20GB ?
Where are these files + folders located ?
Are they essential or useless junk ?
This seems a bit strange .
Any advice is appreciated .
Thanks in advance .
MacBook Pro (15-inch Late 2011), OS X Mountain Lion (10.8)