What error message do you get (exactly) when you try to boot from this one?
My inclination would be to "clone" it, then try to repair the clone. I've had good success in the past with the install DVD, not in "repair" mode but in "fresh install". It then finds the previous install and asks if you want to fix it. for some reason, that works a lot better than the repair option (for me, anyway).
But if there is a load of critical stuff on the original drive, I'd not risk that on the original. A clone would/should do it.