Question about Carbon Copy Cloner

[Shanegrla]

Hi everybody,
I'm looking to get PT 8 and I need to update to Leopard, so I wanted to backup my system drive first like they say to do using CCC (the plan is to b/up the system drive to my external Glyph drive, the one I put all my PT sessions on). So I'm just about to hit Go when I see a little note at the bottom of the screen that scares me a little bit:

Items on "Glyph" will be overwritten by items at the same path on "Macintosh HD".

Does this mean it will erase everything on my Glyph drive (including all my sessions) and replace it with the copy of my system drive? Do I need to be doing this on an empty external hard drive? I hope not. I don't wanna be spending another couple hundred bucks on a new drive just for a backup. I'd rather be spending my money on ProTools goodies, etc.

Sorry, I'm not a genius when it comes to computer terminology. Maybe someone can clarify this for me? Thanks.