Ubuntu 12.04 installation wnt wrong, partial install on wrong hd

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http://askubuntu.com – I attempted an install of 12.04 on my main pc. I mistakenly left-on the power to my 300G external drive. During the install for some odd reason it began installing to the 300Gb external with all my important files on it. When I seen the light glow on the external i immediately canceled the install. In retrospect I have 298Gb free on the drive with 3 empty folders: etc, lost+found, and media. All my former data isn't accessible (of course) and only shows these 3 folders. Question: How can i get my drive back to its former state; un-install the install; or recover my data? (HowTos)