As a new user who tends to mess around with things (I currently have three
drives running four operating systems) I've had this problem many, many
times.
There's an 'Ubuntu Boot Repair' disc/program that has been invaluable to me.
It can be found at: https://help.ubuntu.com/community/Boot-Repair
I recommmend burning a copy AND installing it on your system. (Just in case.
It never hurts to have a copy handy.) When it's done it tells you which hard
drive to boot from.
This program is pretty much idiot-proof. Trust me, I know. :)
Note: Boot-Repair is free software, licensed under GNU-GPL.
