My macbook says it is almost full. How do I test if this is true?

Corrupt index on your hard drive. There must be an applet that you run to fix it. In Windows it’s called checkdisk.

perhaps it has been midnight snacking, and this has subsequently thrown off its eating schedule?

try feeding it a cheeseburger, and if it doesn’t go for it, than thats your problem!

(seriously, get an external hard drive, and move unnecessary files, and applications onto it…)

First. Restart and see if that gives you back your space. Let me know if it does and I will explain what happened.

Second, run GrandPerspective and see what is using it all.

Open a finder window, navigate to your home folder (command+Shift+H) and select the desktop, documents, downloads, pictures and movies and music folders (hold down the command button and select each folder with the mouse button). When they are all selected press command+I. This will bring up info windows telling you the relative size of each folder. If you notice one that appears much larger than you think it should be, close down all of the information windows and open up a finder window for the folder that appears larger than you expected. Select the it’s subfolders and perform the command+I function again, either individually or en mass. There is a good program called Whatsize that scans your hard drive and tells you where the biggest files are, but it is shareware and only let’s you scan 20gb for free (13 bucks will remove this cap, but that’s kind of steep for something you can do yourself with minimal sleuthing). Also, check to see that you don’t have doubles of all your music. If you are in the habit of putting music on your computer in someplace other than directly into the iTunes music folder, iTunes will make a copy of the file in the iTunes music folder when it is played (i.e. it will copy the file rather than simply move it). However, MAKE CERTAIN that the itunes folder contains actual copies and not simply aliases. Of course, this goes for pretty much anything you delete. The best rule of thumb is to backup your computer to an external drive BEFORE you start deleting stuff using time machine or a similar back up program. Good luck.