Where is all my memory hiding?

Question

My daughter has an older Powerbook 4 whose hard drive had been partitioned. One volume called "Data" says (under "Get Info") that it has a 29.44 GB capacity of which only 2.23 GB are available; but the volume "DAta" has two main subfolders on it, and they use up only 15.38 GB. So, where are the 11 or 12 GBs that I should have access to? I could really use them, if I could find them or erase something that would give me access to them.
What am I missing here?
Thanks

Answer 1

OK, I just took a look at my own drives and I might have a better answer now.
On my main drive, using the Inspector (hold down Option while choosing File > Get Info) I get:
1. 201GB used when the drive is selected.
2. 193GB used when all the folders in that drive are selected.
In this case, the difference is due to all of the invisible system folders and files that you can't normally see in the Finder. Mac OS X hides those because normal users are not supposed to touch them and should never be allowed to alter or delete them, especially by accident. You can see them in Disk Inventory if you are using the split view with the colored rectangles on the right and the folder list on the left. You can see many more folders and files at the top level of the drive than are visible in the Finder, such as "etc" and "private".
Looking at my disk in Disk Inventory X, I see some huge rectangles and want to check them out, so I click some. One turns out to be 5.1GB of a single iMovie project that I haven't finished and forgot about. An 8.5GB square is my Parallels disk image of Windows XP, not much I can do about that. A 7GB invisible file inside private/var/vm is called "sleepimage." Ah, that's the temporary system file that stores data for Safe Sleep, I believe. I also see a folder called .Temporary Items/folders.501/Cleanup At Startup/PhotoshopTemp84739573849. That should go away when I quit Photoshop.
Now about deleting these invisible files. You usually do not want to. They are invisible for a reason. They are managed by OS X. Unless you are enough of an OS X engineer to know what you are doing, deleting or altering invisible temp files may destabilize your system in a hurry. The system and the apps do not expect these files to be manually manipulated. If they go to get data out of them and they are not there, say hello to a crash, potentially. Many of them go away on their own when you close programs or restart.
This is why if you are running low on disk space, don't bother the system, instead look for huge visible files or folders that you can clean up, like that 5.1GB iMovie project I forgot about. One time it was the render files for a Final Cut Pro project taking up many GB. Another time I decided my iTunes folder had so much music on it that it was time to move that 30GB to another drive.
Sometimes you feel that you do not want to move anything off the drive or can't find any fat to cut out. Sometimes that means you just need a bigger drive in there. I put in a 120GB drive because I wanted to maintain enough headroom for all those files that do come and go on their own, knowing that it's a bad idea to see free disk space fall under 5GB. I shoot for 10GB free myself.