I have had a similar issue, so i thought i would share my experience, perhaps helping someone else with similar..... (As the above posts helped me to understand what had happened)
A few weeks ago I decided to install ccleaner on my mac (osx).
(I use pc's mostly and find ccleaner one of my top tools.)
Then one recent evening I ran the 'erase free space' tool on my mac, single pass, and off it trotted....
all was well. As it was taking a while, I left it running and went to do something else. (My mac shuts down automatically after 3 hours...)
The next day...Start up disk is full warning! , looked around the system, emptied the trash – still no space. So I opened finder, 'cmd+I' on my hd to find I only had 40GB left on my 500GB drive.
I then searched on google for answers, came across this thread, searched a bit more on the old t'interweb..and read several articles that sorted my issue. I now have my missing 250GB of space back.
I'm afraid to say ccleaner will be removed from my mac in favour of Omnidisk Sweeper (I will keep ccleaner for pc though)
From what I have read, this is what I think happened:
1. Ran ccleaner (Erase free space)
2. Sometime after me leaving my mac - my mac shut down part way through the scan
3. The temp file that ccleaner created must have been the 250 GB hidden in the trash can and I couldn't access it to delete it.
4. I installed omnidisk sweeper, ran it, but, it didnt give me the space back or identify the file, then I read this article:
Link to using omnidisk from terminal (superuser)
then hey presto! 250GB of space loveliness reinstated from the black hole of the trash can.
I will try not to be as naïve in future thinking that all software is the same across platforms
and please read the warnings about using terminal if you are a novice.
...and i too would like to curl.
Edit: please don't get too pedantic about the numbers not adding up as i wrote this by memory.