no one can use "very full" drives. I dont care if they are SATA, Firewire, PATA, or SCSI. People use every byte of a 500 gig drive, and start to cry. It's the SAME old answer - SPEND SOME MONEY and buy a new drive. If you "splurge" for a 1 TB SATA drive, when this drive gets "full" you will face the same problem. When you are single, your studio apartment is ok. when you have 3 kids, your studio apartment is no longer OK. You eventually commit suicide. SO, before you do this because of your problems, buy another drive. Besides, I know that you have got that fat belly, and you want to lose weight, so stop eating that ham and turkey for christmas, and buy another drive. Your FCP system will be happy, and your behind will be happy.
Try Prosoft Drive genius fro drive maintenance, and also a freeware app Grand Perspective. GP will give you a graphical representation of your files according to their size, and then you can click on the "square" on the graph to isolate a specific file in Finder, then delete it. It goes into hidden files as well. Works on OS volumes, but don't delete anything strange unless you are sure about what you are deleting.
PS - don't sweat Our Man Crazy Bob Z. No one's better. Install a filter or something.