This is from the iPhone Dev Team Blog. Sorry if this was already posted or if you have already seen it on their blog. i'm just trying to help others out because i had the same problem and i fixed it using the dev team's method. here you go. Fixing DFU mode on 10.5.6 As noted previously OS X 10.5.6 introduced a bug that affected the use of DFU mode. with some Macs. There have been previously published hacks and techniques to fix this, but here is another method that can be used to temporarily restore DFU functionality in order to use QuickPwn or PwnageTool. 1. You will need an account with ADC (Apple Developer Connection) this is free and takes a few minutes to sign up, you should read the terms and conditions carefully and you should only sign up if you are thinking of developing applications in the future - http://developer.apple.com/mac/ 2. Download the disk image “IOUSBFamily-315.4-log.dmg” for Mac OS X 10.5.5 Build 9F33” (yes, that is a “5” in 10.5.5 - this is a developer debug package of the USB kernel extension). 3. Unplug non-vital USB equipment, such as external DVD writers, USB scanners, USB mass storage devices, at the most leave a Keyboard and Mouse connected. 4. Install IOUSBFamily-315.4.1.pkg from within the disk image 5. Reboot your system! 6. Perform necessary DFU activity with QuickPwn or PwnageTool. 7. Download the disk image “IOUSBFamily-327.4.0-log.dmg” for Mac OS X 10.5.6 Build 9G55” 8. Install IOUSBFamily-327.4.0.pkg from within the disk image 9. Reboot your system! 10. Reattach your USB peripherals.