DFU means Device Firmware Update. If a restore using Recovery Mode doesn't work you will want to use DFU Mode as a last resort. When placing your iPhone into DFU Mode it does not load the OS before attempting the restore

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To put the iPhone into DFU mode so you can do an iTunes firmware restore follow these steps:

Step One
Open iTunes and connect the iPhone to your Mac.
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Step Two
Press and hold the Home button and the Sleep/Wake button at the same time.
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Step Three
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
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The iPhone screen will remain black like this:
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NOTE: It may take a few attempts to get your iPhone into DFU mode. Generally, I hold down both buttons then release the Home button just before I think the Apple logo would appear. If you are still holding both buttons down and you see the Apple logo you are holding them down for too long!