How do you compute from Exterior HD on a public Mac

[piatti]

I noticed that the Geniuses at the Apple Store tested to see if something was wrong with my hard drive by running the MBP from their own hard drive. I was wondering how one does that; I would like to do that with the public computers.

 

[SDub90]

you install an OS on the external hard drive - exactly the same way you would on an internal drive, then while the mac is booting, simply hold down the "option" key and it will show you all bootable drives attached to your mac.

on a PC, you would have to interrupt normal start up and select a different boot device (how this is done is different for each manufacturer).

 

[piatti]

How do you do it for Dell computers?
How do you install OS on an internal drive?