My iMac suddenly went from 9 GB free to 100 GB free space. Can't find what is missing?

Question

10.6.8, late 2009 imac, parallels, Win XP, 1TB hd. Can't find anything missing. Have compared with Time Machine backups, have clicked on various files to verify they are still available.  Haven't verified every file, but all spot checks show files still there. A few months ago HD started running almost constantly for 1-5 mins when Finder is open (ran Activity Monitor) and imac slows way down, almost unusable till hd access stops. Now suddenly the free space has gone from 9 GB to 100 GB. I've run with 8-20 GB free space for 2 years. Disk util shows no prob. Where did extra 90GB come from? Where can I find out what might be missing?

Answer 1

Bob, this has taken awhile, but I've done all you suggested.
I did do 2 Apple software updates just before the 9 gb of free space turned to 100 gb. I remember there was a Safari update plus I finally did the iTunes update that had been sitting in the update list for months. Neither required a reboot, which surprised me, but I did a reboot anyway. My entire Music folder on my hd was about 32 gb after the update and looking at the Time Machine backups was about the same before the update. Is it possible there were huge hidden files, caches, etc associated with it that may have gotten deleted with the reboot?
I looked at the sizes on all my Time Machine backups over the past year up through the ones I did immediately after the free space change and one I did 5 days after the change. This is where I was totally SHOCKED. All of them show my Mac HD about the same size as it is now - 899 gb! Even tho Finder showed only 8 to 20 gb free during the past year, Time Machine's backups were 880 to 899 gb in size, basically 100 gb less than what Finder showed. And they're the same size now. So I'm thinking you were right in your original thought that some huge swap, temp, etc files suddenly got deleted???
I checked actual folder and file sizes and contents currently and in the Time Machine backups before and after the 100gb change. Also in backups several months ago. All are similar or reasonable. So I don't think any of my data is missing. Whew!
I wondered if something (not data) in Parallels using Windows XP was deleted. Over the past month there have been many hangs or sudden closing of a few Windows programs. I know these create dumps of data, and looking in Windows I only found a couple dumps, but they were very small in size. Don't know if this could have totalled 100 gb tho. I close Parallels or reboot Win XP frequently, always after probs in it. I close it also before doing TIme Machine backups. I did just do a TIme Machine backup with it still open, but there was no difference in the size of the backup file. Is it possible temp, dump, etc files were held by WinXP over the past 2 yrs, then suddenly deleted? The timing of this with WinXp no longer being supported did pass thru my mind also. I have Norton antivirus in the Windows side of my mac, so no virus warnings, tho I did think of that too.
I also spent time looking (not touching) the tmp, log, etc files you showed me how to get to. Very interesting. I am actually surprised my computer is still working after looking at all the error msgs in the logs.
Thank you for all your help and suggestions, Bob. I'm confident my data is still ok. So now with all this extra space I can update Parallels and then update to Mavericks. Does that sound reasonable to you?