iMac won't recognize the external hard drive.

[JukeboxJerry]

I'm getting ready to upgrade to "Snow Leopard", and want to back up everything that I have now. I just found that the external hard drive (that was purchased with the iMac in 2009) has never been backing up anything. The message for "Time Machine" is "A storage location for Time Machine backups isn't set up." In the "Set Up Time Machine" option, when the slider is moved to 'on', there is nothing listed to "Use for Backup". The USB ports all work, and, the hard drive is recognized on my laptop. Any suggestions are appreciated.

 

[chscag]

What format is the hard drive using? The only format that Time Machine will back up to is the Mac OS Extended Journaled file format. (HFS+)