Can Ubuntu be installed on external drive and be made selectively bootable?

Hi All

Wondering if Ubuntu can be successfully installed on an external USB drive AND made selectively bootable? By this I mean on a Mac, to choose which drive one boots from at startup is achieved by pressing the 'Alt' key during startup when you're then presented with the bootable volumes connected to the machine to choose from.

I have a 160GB external USB HDD and am wondeing if this type of installation is possible with Ubuntu?

Thanks