On boot press f12,usually the one,this gets you to set-up,then select boot order,mine is set to boot from cd drive then hard drive C,also make sure in your computer sees said drive.
Bye the way you do mean a external hard drive,not Dvd drive
As to read from external usb or Dvd-cd,they will write to memory,as in some linux etc pared down Os's,where's a full Os needs to write to hard drive.I fink?