My 3 year old Mac is having trouble and freezes up from time to time lately. I get a message saying "startup disk almost full, delete files ..." something about having enough room for applications to function. I know my startup disk is the Macintosh HD, but what kind of files do I need to delete and how much space do I need to free up? In my "about this mac" area, it says I have 4.89gb free out of 319.21 So I am confused why I need to do this.
You're less than 2% free and should always have at least 15% (my SWAG). Best is to get an ext FWHD and move stuff from the startup disk to it and delete it from that. Best things to move are movies, music, and photos, using the apps that manage those things.