The iPhone's Reset function allows you to return your phone to its original state and is an ideal way to prepare a device before selling it or giving it to a colleague. However, it is wise to keep an eye on your iPhone as it resets because the device can occasionally hang during the process. In addition, different models of the iPhone take different lengths of time to reset.
Ensure that any iPhone data you want to keep is backed up before resetting. To reset your iPhone, go to the "Settings" menu, tap on "General," then tap on "Reset." Finally, tap "Erase All Content and Settings." After the reset has begun, the device will be inoperable and should not be interfered with or powered down. Ideally, you should perform the reset with your phone plugged into an outlet, as this will eliminate the risk of its battery running out midreset.
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The expected time taken for the reset process to complete varies with both the model and storage capacity of your iPhone. The original iPhone and the iPhone 3G can take several hours, as they are designed to write blank data over your old data after it has been erased as a security measure. However, iPhones from the 3GS onward use encryption instead of overwriting to protect your data. As this process is much quicker, these phones should reset in a matter of a few minutes.
If you are unable to reset your iPhone through its menu, you could try the "Restore" option in iTunes. Restore works in much the same way as resetting your device, returning it to factory default settings. To use Restore, connect your iPhone to your computer and select your device when it appears in iTunes. Click the "Summary" tab, then click "Restore iPhone." This will cause the iPhone to reset itself, with the process taking a similar length of time to that of a regular reset.
If your phone crashes during a reset or restore, you may be able to get it working again by placing it into recovery mode. To do this, turn the phone off by holding the "Sleep/Wake" button down and sliding the red power slider. Next, hold the "Home" button and connect the phone to an computer with iTunes installed. Continue to hold the button until the "Connect to iTunes" screen appears, then open iTunes on your computer. The phone should now show in iTunes, allowing you to attempt to restore it again.