I know I'm weird, because I get a little thrill out of seeing new error messages in iTunes. Most people groan, and granted it takes some time out of my day, but I get to make a new blog post out of it and hopefully help a bunch of people! So without further ado...
This morning I'm testing some iPhone drive cleaning software and restoring a cleaned-up backup. While doing so, I got this message:
Considering that my iPhone USB cable has electric tape holding it together, I first assumed that my USB connection to the computer was faulty, and unplugged everything, blew on everything, and plugged it all back together. When that didn't help AND I noticed that my phone was actually still connected to iTunes, I realized that "because the iPhone disconnected" means more specifically "because the iPhone disconnected from the restore service." Since I'm intentionally editing my backup (to reduce the file sizes,) I could deduce that there is likely an issue with the backup that I am restoring.
If you are seeing the message "iTunes could not restore the iPhone because the iPhone disconnected." it likely means that the iPhone restore process is crashing on the iPhone. Possible causes and solutions:
Your iPhone needs to be rebooted. Since we're getting a crashing process, the easiest thing to try first is to reboot the device. This will fix any simple issues that could be causing crashes on your iPhone.
Your iPhone backup is actually corrupt. Unfortunately, iTunes and iOS support for corrupt backups is not very good. We've made some tools to help though! You can repair the backup using our Decipher Backup Repair so that you can restore the repaired backup in iTunes. Or, you can save data from your iPhone backup with Decipher Backup Browser. You can also leave us a comment and we can discuss how we can help!
Something we don't know yet! If you're getting the message that your iPhone disconnected during a restore, and are feeling adventurous, we would love to have you view your iPhone's debugging log so we can find out more causes for this problem. Click here to learn how to view your iPhone debugging log in the iPhone Configuration Utility, try to restore again, and then leave us a comment with your debugging log contents. We'll try to help as best we can!
Update April 17, 2018: We originally started seeing the iPhone disconnected error back in 2014, and since then we've done a lot of research and fixed a lot of broken iPhone backups. You can read a more recent blog post about the "iTunes could not restore the iPhone because the iPhone disconnected" error here.