Some basic hard drive questions.?

I have an desk top dell computer (about 5 yrs old) that stopped working. After the 6-7 magnitude earthquake last wk in socal (not sure if it's coincident or earthquake shake caused it), whenever I turned on, it keeps saying unable to find hard drive. All the cable seems to be ok and i suspect hard drive went... show more I have an desk top dell computer (about 5 yrs old) that stopped working. After the 6-7 magnitude earthquake last wk in socal (not sure if it's coincident or earthquake shake caused it), whenever I turned on, it keeps saying unable to find hard drive. All the cable seems to be ok and i suspect hard drive went bad. I want to see if it can boot by connecting external hard drive to USB (which I've never done before). I was reading online and it says external hard drive has to have bootable OS in it. I tried to search which external drive have this bootable OS. but, online search didn't do much help.

Which external drive have this bootable OS feature? or is it that none of external drive come with this feature and this is something I need to modify the external driver for? My computer is window 7. will it be too difficult to try to boot it just by external drive and it's better to just buy new motherboard? thanks.
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