Obviously something is filling up your drive.
Not sure how you counted up all the folders on your drive, there are thousands of them and a great many are invisible.
Did you only count the ones in your Home folder?
You save nothing to that drive? No iPhoto, iTunes, iMovie, documents?
I'm not sure this is the correct category for this question but here goes:
While doing some work on the mac pro a warning message popped up saying that my startup drive ( Macintosh HD ) was nearly full. I never save anything to this drive so I was a bit confused.
Upon looking at the size of each folder on the drive, I found that the total added up to about 42 GB. The capacity of this drive is 320 GB yet the info panel for the drive is saying that 318.87 GB have been used. Take a look at the link to screenshot jpeg (I've not used this forum before and neither html nor UBB code seem to work to embed a jpeg image). Am I missing something obvious?
I've run Disk Warrior on this drive and it repaired 'critical values in the volume information'. Yet the drive remains mysteriously full.