use a dedicated 12v power supply. Look for psu for led lighting - same thing but more power and cheaper.

Also having 12v go into a usb socket should only fry that particular usb chip.
How old is the computer ?
Your actual harddrive should be fine - motherboard might have been fried, but In 30 years of building and fixing pc's I've only twice seen a fried harddrive and they were from malfunctioning psu's not input from shorted out usb.