Hello, I am trying to install windows 7 on my imac 21.5" 2010. My DVD drive doesn't work, and since I must one in order to install my Wndows 7, I bought an external DVD drive. Everything went smooth, but then suddenly it said that message: No Bootable device. Insert boot disk and press any key. I think that it caused because it doesn't able to read my external DVD drive when it installs the Windows 7, but can only read internal's one. What should I do in order to fix that problem? Thanks in advance, Daniel.