Re: Don't know what to select in "Device for boot loader instalation"
If you are installing Ubuntu on /dev/sdc (Exteranl HDD) then it will be better to install Ubuntu GRUB (GRand Unified Boot-Loader) on /dev/sdc only and NOT /dev/sdc1 or sdc2 etc. If you install it on /dev/sda (which I assume is your Mac) then Grub will overwrite the Mac bootloader. I don't use Mac so I am not sure if rEFIt will detect Ubuntu or not.
If for some reason rEFIt is not able to boot Ubuntu then you should look in BIOS or whatever and tell it to boot USB devices first (assuming your External HDD uses USB), so when ExHDD is connected your BIOS will boot it first if not you will boot into Mac.
"Evolution is Nature's way of issuing upgrades."