If you need to do a clear install of OS X 10.9 Mavericks without resorting to the Internet Recovery or Recovery HD partition, here is how. You will need the “Install OS X Mavericks App” from the Mac App Store, a free 8Gb+ USB disk and your Terminal app.
Make sure you erase the USB disk and change the partition type to Master Boot Record. Leave the default “Untitled” name for the simplicity (or replace all occurencies of it in the following command).
To enable the dual screen support on a multi-monitor Hackintosh equipped with an AMD Radeon HD 6870 under OS X Mountain Lion (this works with the current 10.8.4 too), enter the following shell code into your Terminal session: