How the portable hard disk is used as ua bootable disk?

Answer 1

portable hard disk usually identifies as d or e depending on how many drives are in the computer and boot order is changed to reflect external device first in cmos.

Answer 2

Some computers are able to use USB devices, such as Hard drives or memory sticks, as the device from which they boot. You can see if your computer is able to do that by going into the computer's BIOS set up. Check in the Boot Order options and see if it has "USB device" as an option. You would need to move USB Device above Hard Drive in the boot order. Then it will look for and boot from a USB Device before it looks to the computer's hard drive.

The USB device would have to have Windows, or some other OS installed on it to make it bootable. You can install Windows onto a USB Hard Drive by booting from the Windows CD and after you do the F8 to agree, selecting the USB hard drive as the location were you want Windows to install.

Normally you would not want to run your OS from a USB device. It is going to be slower than an internal drive would be. You would only use this as a method of trying to access a PC if the internal hard drive was not working. You would boot from the USB device, and then hope you can open and salvage data off the damaged internal drive. It is not intended for running you PC on a regular basis.