How do you compute from Exterior HD on a public Mac

i noticed geniuses @ apple store tested see if wrong hard drive running mbp own hard drive. wondering how 1 that; public computers.

 

you install os on external hard drive - same way on internal drive, while mac booting, hold down "option" key , show bootable drives attached mac.

on pc, have interrupt normal start , select different boot device (how done different each manufacturer).

 

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