Q: iTunes could not back up the iPhone because the iPhone disconnected.

Same here with iPhone 4S - no matter if connected via WiFi or via USB. All media (photos, apps, music) syncs nicely, but backup fails. Oddly enough, everything works well with my iPad Air 2.

I have the same problem.  Seven days ago I backup my phone with no issues and today that's a different story.  It starts backup but eventually stops and says it cannot backup because the phone is disconnected.

All OSX on phone and computer is latest.

I found another site that said to reinstall itunes.  Done, with no change.

I found yet another forum that said delete backup files, they are corrupt.  Done, with no change.

Another forum said the Genius Bar folks say its a software problem and fixed it but none of the "how to" arrived on this forum.

Any help?

Ok.

Step 3 and 4.  Dont know which one did it but it finally worked.

I changed my iphone name and turned off the wireless and phone service.  It backed it up finally.

I have been backing up iphones of mine for years and this is a first.

This step is the fix.  To clarify, all I needed to do is turn off WiFi and Cellular services.  I did NOT have to change the phones name. 

Worked like a charm, thank you for your help.

We were getting the error message that said "iPhone could not be backed up because it has been disconnected." We followed all 6 steps in Apple's tech support article, If you can't make an iTunes backup of your iPhone, iPad, or iPod touch - Apple Support, but that didn't fix the problem for us. However, the thing that DID fix the problem for us was trying a 7th unlisted step: We deleted all previous iTunes backups by going into iTunes > Preferences > Devices, and deleting all previous backups. Then our backup worked after that. We also reported this information to Apple, so hopefully they will update that tech support article as well.

I THINK I HAVE SOLUTION THAT IS REPRODUCIBLE 

I have this problem for months and have tried everything, including factory resetting. I have removed apps. I have changed wires, computers. Resintalled software. changed every setting there is. Followed advice from every forum. Called customer support. Somethings temporarily worked. Many things didn't work. So you get excited and then get ******. I have no idea why this bug happens.

Found the one thing that FOR ME consistently works. May not work for you. But I've done this a half dozen times now. I have phone plugged into my iMac. (I haven't been able to consistently get wifi backup to work). iTunes is open. I then, while plugged in, do a restart on the phone (home + power). phone asks if i trust computer and I need to enter password key. Then I do an encrypted backup. Seems to only work when i hard restart while plugged in.

I hope this works for you.

Thanks dude. This was driving me crazy. At least for this one time, it worked. I wish apple would fix this any time soon.

Thanks again!

I also get the error message about iTunes not being able to back up iPhone because the iPhone disconnected, and the ONLY way I can back up is to go into iTunes preferences, choose devices and delete the last iPhone backup. Then, you can back up as normal UNTIL the next time - whereupon you have to go through the same rigamarole all over again. I am really sick of this and Apple Telephone Support does not know of a permanent fix. What a pain...so un-Apple like. Steve would never have stood for this.

Apparently possibly maybe... the iOS 9.2.1 update solved the problem??

Just did an incremental backup over WiFi (...!) - and it worked flawlessly. 

Would be too good to be true...

Nope. Still happening in 9.2.1. No surprise really. This issue has been there for many years now.

That's weird. Tried again five minutes ago - still working (iPhone 4S 16 GB [3 GB free space], iTunes 12.3.2.35, El Capitan 10.11.3

SO what is the main fix for this? Everytime you want to back up, delete your OLD backup first? Or just put it in airplane mode?

I hate waiting an hour and then seeing "iphone disconnected". Now what got it to work this time was unchecking the box for encrypted backup. Does anyone actually do that?

Thanks! I turned my phone on Airplane Mode and voila, no more error message and a clean backup.

This is all I did (deleted backups) Edit -> Preferences -> Devices and it worked.