I am now trying to install Ubuntu on a Mac. My plan is that when I have it installed I use rEFIt to select the operating system I want to use (Mac OS X/Ubuntu 12.10). An IMPORTANT thing you should know is that I am trying to install Ubuntu in a previously made partition (made with Disk Utility on Mac) IN AN EXTERNAL HARD DISK DRIVE.
But my problem is that, when I'm installing i am asked to select the Device for boot loader instalation and I don't know what to select. I am also worried that I replace the Mac's boot loader with Ubuntu's boot loader, and I don't want that because I don't have the Mac's boot disk. I get 4 options in the instalation process when selecting "Device for boot loader instalation":
-/dev/sda *numbers and letters* (this is the internal HDD)
-/dev/sdc WD*numbers and letters* (External HDD, this is where I want Ubuntu to be installed)

The question is: What do I have to select?