Why does iTunes fail to sync my iPhone? For the past month or so, I've been unable to backup my iPhone using iTunes.
Every time I try, I'm told that the phone is "disconnected":



What could be causing this?
I've tried different USB cables and different USB ports, but the result is always the same.



I'm using OS X 10.10.3, iTunes 12.1.2.27, and iOS 8.3.
The procedure I'm using is the obvious one: launch iTunes; connect the iPhone; choose the iPhone in iTunes; press "Sync".
If you've tried all physical options, removing hubs, changing cables, ports etc, then the remaining likelihood is that the existing backup is corrupt.

Simplest method would be to find your backups folder &amp; rename/delete the corrupt one, then test again…


Disconnect phone, quit iTunes
Switch to Finder
 Cmd ⌘   N  for new window
 Cmd ⌘   Shift ⇧   G  for Go To…
Copy/paste ~/Library/Application Support/MobileSync/Backup/ &amp; hit  Enter ⌅  or  Return⏎  
Change the name of the folder with the long, complex number - just add .bak or something to the end of it. (If there are multiple folders, see the note at the end)
In iTunes, try a new Backup, then Sync.
If successful, you can delete the old .bak folder


If you have multiple backup folders…  


If you only ever backup one iDevice to iTunes, rename all the folders the same way, add .bak etc
If you backup multiple iDevices, you can find which is the correct folder by matching the device identifier with the backup folder name by clicking on the Serial Number field in iTunes (which will switch it to your device identifier.)