166Gb of free space not showing in Finder

Question

Hi, my Mac started saying I need to free up space on my Startup Disk. I was suprised, but as I've been doing some film work, I just thought it must take up even more space than I thought. Anway, I free'd up loads of spaced, I moved all my film files to an external disk, deleted them from where they were, and emptied the trash. I've got rid of other things like Internet cache and downloads, basically, I know i have space now. However, my Mac insists on saying I only have about 2Gb free. On another site, someone recommended something called Disk Inventory X, which gives a visual picture of what files etc are taking up space on the Mac. I ran this, and low and behold I have 166Gb free. The screen shot below shows the big blue square of free space. If you can zoom in, it shows this as something outside of the home 'Tiger' thing, under Users - .tiger, as though its partitioned or something?! I don't really understand how Mac's work under the hood of pretty buttons etc, so I'm a bit stuck now. 
Can anyone please tell me why my Mac can't see or let me use all this free space?
I am running Snow Leopard 10.6.8. It is a 13" mid 2009 MacBook Pro 5.5, 2.26 GHz Intel Core 2 Duo with 4Gb 1067 MHz DDR3 if that's of any help.
Many thanks in advance.

Answer 1

With regard to corruption at the HD level, it is unlikely but possible - I would run Disk Utility's "Verify Disk" if you haven't already done so.
'It' is the Home folder in Finder
Yes, as I mentioned in my earlier post, it is "normal" (and not meaningful) for a FileVault Home to show an apparent capacity that is twice the capacity of the HD itself. The "Used" space reported there is likewise meaningless - that's why I earlier suggested a different method for determining the total size of your user files. However the reported "available" space is apparently accurate, and at least in my test setup is the same number that you will see for "available" if you Get Info on Macintosh HD.
I have 177Gb of User stuff, this also includes my Applications
So as not to compare apples and oranges, "user stuff" generally refers only to those items within the home folder. Some people have a second Applications folder within the home folder, but if you are instead referring to applications in the standard Applications folder (HD>Applications) then that space is generally not counted as "user files." It makes a difference when you are trying to see if the numbers "add up", as is the case here. In your first screenshot, Disk Inventory X reported 94.5GB in HD>Library, 23.3GB in HD>Applications. and about 13GB in other places, for a total of 131GB, which after the 7% correction to decimal GB, is about 140GB. If you have 177GB of stuff within your home folder using the method I suggested earlier, then that would completely fill your 320GB HD. If you had counted the entire main Applications folder as part of your "user stuff"  then you would be expected to have about 25GB free - still not a lot on a 320GB HD.
With regard to just wiping the HD and re-installing the OS, yes you can then drag back your user data, but apps can be problematic. Apps that were originally installed by dragging them directly from a downloaded .dmg or apps that were bought in the App store can usually just be copied back into /Applications, but apps that were originally installed via an installer package often have support files that were installed in other places as well, and such apps usually have to be re-installed from scratch. The Libraries generally contain important information as well (Mail, Keychains, and Bookmarks as well as application support files and preference files). You would also have to re-do network settings, etc.
I asked whether you had a large (>750GB) empty partition of external storage still available, because I think that would be  your best bet for recovering everything intact. If you have such a large external partition formatted for the Mac, I would try the following:
Create a new admin account, log out of the FileVault account, and log into the new account. From there, run Carbon Copy Cloner to clone Macintosh HD onto the large external drive. Reboot from that external drive, and log into the FileVault account there. Now everything should be as before, except for one important  difference - you should now have enough free space to turn off FileVault! If this works you could eventually clone back to the internal HD, presumably after pruning back some of your stuff if necessary.
FileVault, as you have seen, can cause lots of issues. I have never used it except for testing and tinkering. It was completely re-done for Lion, and Lion's FileVault is reportedly is a big improvement, but I haven't used it there myself.