18 GB of phantom hard disk space -- how do I find out what's using it?

Question

My hard disk is full.
Only not really. Its total capacity is 148.6 GB. According to all publicly viewable sources (disk utility, etc) I am currently using 147.3 GB of that. Therefore I decided to examine every item the hard drive to determine what to erase.
My user folder is using approximately 96 GB of disk space. There are no errors here; I added up the subfolders and they do indeed come to approximately 96 GB (about 76 GB of this is music, thus the primary drain, which I'll probably transfer to an external disk in order to free up space). The other, system folders -- applications, library etc -- take up approximately 33 GB of disk space. I probably won't touch these save for applications I don't use, for fear of breaking something.
Put together this is approximately 129 GB. There is 1.3 GB listed as free. What happened to the rest?
What's using it? The trash is empty. There doesn't seem to be anything else on the hard drive to contribute to a space drain -- I totaled up every file and folder shown under the Macintosh HD view. This is a 17 inch MacBook Pro, from 2007, hard disk replaced in mid 2009 (it thus may be nearing the end of its natural lifespan anyway). At this point I would attempt a backup, format and restore but I don't know whether that would also wind up restoring whatever's using up the missing 18 GB. Or indeed whether this doesn't indicate a problem with the drive itself that formatting won't fix.

Answer 1

You can get rid of the sleepimage file if space is that tight, run Terminal and use the following:
To see or set the hibernate mode use the command
pmset -g | grep hibernatemode
Results:
0 - Old style sleep mode, with RAM powered on while sleeping, safe sleep disabled, and super-fast wake.
1 - Hibernation mode, with RAM contents written to disk, system totally shut down while “sleeping,” and slower wake up, due to reading the contents of RAM off the hard drive.
3 - The default mode on machines introduced since about fall 2005. RAM is powered on while sleeping, but RAM contents are also written to disk before sleeping. In the event of total power loss, the system enters hibernation mode automatically.
5 - This is the same as mode 1, but it’s for those using secure virtual memory (in System Preferences -> Security).
7 - This is the same as mode 3, but it’s for those using secure virtual memory.
To set (for example 0) use the command
sudo pmset -a hibernatemode 0
The delete the sleepimage file using
cd /var/vm
sudo rm sleepimage