I am currently in the possession of an iPhone 3G that keeps looping into DFU, unable to restore using 3.1.3 IPSW nor 4.2.1 IPSW. The owner prior to this had tried restoring it a month ago, still to no avail. He asked me to unlock it for T-Mobile, and I agreed. Everytime I connect the iPhone into my computer (Mac or PC), iTunes tells me it found an iPhone in recovery mode, etc. I shift/option-click Restore, find 3.1.3/4.2.1 IPSW, open it, and error 21 pops up and the screen turns white. I have tried numerous tutorials on error 21, none of them have worked. Some have had a 'Recovery Loop' where it just shows the Connect to iTunes image, but my screen is completely black, and TinyUmbrella recognizes it as a device in DFU. Please help! I've tried both my Mac and PC, two different cables, four different USB ports. Nothing.