I am trying to format my new Buffalo MiniStation Stealth for my mac 10.7.5. I followed all the steps in the manual for formatting, but every time I click apply for the partition, it says "Couldn't unmount disk." How do I fix this?

Computers & Internet - Buffalo Technology - MiniStation - 1tb Ministation Stealth USB 3.0 Portable Hd Black Mac Pc /...