How can I determine the polarity of a power adapter?

I need to obtain a new power adapter for an external hard drive. I have the AC cord but i need the DC to AC adapter. The company won't sell me one and Radioshack needs to know the polarity of the power input on the hard drive to make me a new power adapter. It is not printed on the product, it only says DC... show more I need to obtain a new power adapter for an external hard drive. I have the AC cord but i need the DC to AC adapter. The company won't sell me one and Radioshack needs to know the polarity of the power input on the hard drive to make me a new power adapter. It is not printed on the product, it only says DC 12.

http://www.simpleshare.com/images/sharin...

The power input is the lowest port on the picture.
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