In the past couple of days, we've seen reports here and there of iPhones getting stuck in a restart loop. The black screen with the Apple logo shows up, holds for a while, and then disappears again; repeat ad infinitum. While it's not clear yet exactly what's triggering the issue (the complainants suspect that it may be a recent application update to blame), there is a tried and true way to get your iPhone, iPod touch, iPad or older iPod back to square one: either use recovery mode or do a DFU restore.
As OS X Daily explained, DFU (Device Firmware Update) mode is a means to getting your iPhone to interface with iTunes without iTunes automatically loading the latest version of iOS onto it (it skips the iBoot bootloader, for jailbreak aficionados). Differing from recovery mode -- where iTunes restores your iPhone to its original factory settings but with the latest iOS and firmware -- DFU mode allows you to change and choose the firmware you want to install onto your iPhone. For instance, you would use DFU mode if you want to install a previous version of iOS, jailbreak your iPhone or unlock your SIM. If (for whatever reason) standard recovery isn't working for you to get your device back to working condition, DFU may help you out.
Remember, if you restore your phone in any fashion (via the iTunes 'Restore' button, recovery mode or DFU) you will lose any modifications made to unsynced content or in-app data since your last iTunes sync (game states, voice memos, notes etc.). Be cautious.
So, here's the basic info on how to enter DFU mode (and it works for your iPod touch, too). Instructions may vary slightly by year or model; if these steps do not work, check YouTube's extensive library of how-to demonstrations).
- Connect your iPhone to your computer via USB and launch iTunes.
- Now turn your iPhone off. Hold down the sleep / power button at the top right of your iPhone, then slide your finger across the "slide to power off" button on your iPhone's screen. (If it won't turn off, press Home and Sleep/Power until the screen goes dark, then release.)
- Once your iPhone is off, hold down both the sleep / power button as well as the home button for 10 seconds. Once 10 seconds has elapsed, release the sleep / power button on your iPhone, but continue to hold the home button until the the above message has appeared in iTunes telling you that your iPhone has been detected in recovery mode. (If you just want to go to recovery mode, skip the 10-second hold on both buttons and simply connect the phone while holding down Home, then wait for 'Connect to iTunes' screen on the phone.)
- Click OK. You're now in DFU mode.
It's pretty straight forward, but if you see the Apple logo or the "connect to iTunes" screen on your iPhone, you're not in DFU mode. DFU leaves the screen of the iPhone
completely black turned off.
To exit DFU mode, simply hold down the sleep / power button and the home button together until your iPhone disappears from the device list in iTunes, then turn on the iPhone as normal.
Again, for most purposes (save jailbreaking) there's no practical difference between DFU and a regular restore, except and unless you're in a situation where your phone won't go into restore mode normally. Restore with care!