If you place any applications on the external disk, I would put any third party applications. I would not put any iApp on the external, because when you upgrade any of the iApps, it will be looking for it on the system disk. If you moved all of your mp3s (music) and photos (pictures), be sure to remove them from your home folder//directory. Also if you have anything in the movies directory/folder, move it too.
Accomplish this by..
drag and drop the folder into its new place. Then trash the old folder.
if you have application on the same partition as the OS X system disk, You may want to move some of them, If you have some duplicate applications. It might a good time to do some house keeping. Example you may have a system that has two and three three versions of Toast, and two versions of Final Cut
I need very explicit instructions! I keep getting an error message that says my start up disk is almost full. I am running a Mac OS X!! I'm so frustrated I could cry.
It's telling you that because your.. (don't be shocked) start up disk is almost full. Which is a Bad Thing™. OS X really needs to have at least 1.5GB of free disk space so it has at least a gigabyte to swap (write RAM to disk), and an extra 512 megabytes for some elbow room.
The fix? Delete stuff you don't need. Start with your home directory. Alternately, get an external hard drive and copy stuff to it from your home directory.