Can we boot from an external HDD.

I don't know. Some mobos will boot off USB, some will not, I don't know yours. You could
1) shut down and power off
2) Attach said external hard drive (does it have a bootable partition?)
3) Interrupt the boot sequence and enter the BIOS (is this familiar to you?)
4) Go to the list of drives and see if the external one is recognized
5) Go to the boot menu and see if the external one can be set to the boot device.

and let us know what you see.

Yes My mobo allows Booting from USB.
I dont have any external HDD yet cze I ll buy it only when I am sure its possible to boot from an eHDD.
How to make bootable eHDD, that I dont know and if it works or not.
thnks