So tonight I discovered my hard drive was slowly being choked by some mysterious process writing gigs and gigs to it. So I moved 75GB of files to an external drive only to come back a few hours later and discover my free space was back down to 23 GB.
Where did 50 GB of files come from in just a few hours!?
Rogue process downloading large files...*cough*iTunes*cough*
I had NetUse Traffic Monitor running and it clearly showed that there was definitely not 50 GB of downloads in that time period.
Rogue process writing lots of log files
In my investigation of what to move to the external drive I used GrandPerspective to get a visualization and catalog of what was on my drive. Thankfully I hadn't closed that window so I could rescan and compare what had changed. Here's the next head scratcher, it showed only a 4 GB total difference in used space between the two scans.
It was something on my wife's side which GrandPerspective couldn't see since it didn't have permissions to her files.
A this point I realized that GrandPerspective was reporting 104GB of "miscellaneous used space". So Googling for more info, one of the posts I stumbled on lead me to a page about Mac OSX Tiger Problems which introduced me to the BSD disk usage command 'du'. So it seemed like a lower-level command that would take some time, but would show me what GrandPerspective supposedly couldn't. Sadly it didn't and agreed with GrandPerspective that 804GB of my 931GB drive were being used while 'df -kh' agreed with the Finder that only 23GB were free. So four tools were giving me two different answers, that I had either 23GB free or 127GB free depending on which you asked.
Ask Google why 'du' and 'dk' can give different answers.
Sure enough, Google had an answer to why 'du' and 'dk' can differ. Turns out a process can hold onto a deleted file and the file will be unaccounted but the disk space will still be considered used. So at this point I gave in (sorry, never figured out who the offender was) and restarted my computer. Sure enough, after the Finder came back up...I had magically cleared up nearly 87 GB of hard drive space.
Moral of the story
Try restarting first, even for disk free space issues apparently. #facepalm