writing to ext.hard drive?

I'm a new member and refugee from PC. I have an eMac which has a very small 37gig HD. I can not even download Apple's updates anymore. So I formatted 40gig ext. HD and copied the entire eMac onto it; then formatted the internal HD and installed Leopard on it. What I'm asking: as I said I have the ext. Mac formatted but HOW do I tell the computer to install future data to it? I know OS related data has to go to the int. HD but have a lot to get off it since the OS again left very little space after the install.

Any good explanation will be appreciated.

I'm not quite sure what your goal is here. I take it that you are not replacing the internal drive, but rather added an external drive and wish to use that for storing your personal files?

What you can do is move your personal home folder to the external drive. I'll post a link to instructions, but feel free to post back if you need clarification. I do want to emphasize something. It's extremely important to make sure you have a second admin account on your Mac that you can resort to in case this external drive fails or is not connected for any reason. Otherwise, you will be unable to log into the Mac. You don't have to actually use that account… it just needs to be available in a pinch. Use the directions in the link below to move the user account that you plan to use regularly to the external drive.

TUAW Tip: Moving your home folder to another disk (or moving it back) | TUAW - The Unofficial Apple Weblog

EDIT: actually since you already copied your contents to the external drive, it should be as simple as re-creating the EXACT user name on your new install of Leopard, then following the steps to point it to your old user folder on the external drive. As a matter of precaution, you should try to backup your important data though… just to be sure.