Ahh! det här förklarar saken. Man borde kanske läsa lite noggrannare före
Important note about DFU mode: If you have a newer "unibody" MacBook or MacBook Pro,
or you have Mac OS X 10.5.6 installed on your Mac, iPhone might not go into DFU mode
properly —.instead of InstallerApp logo it will show either black or white screen. In this
case you need to connect iPhone to Mac via USB hub, so iPhone will be able to switch
into DFU mode.