Update - I can finally confirm that I got it fixed. Simply connected my iPhone 4s into one of the main USB port on the BACK of the PC, rather than using the obvious (and convenient) ports on the front panel.
the link I posted above contains virtually every possible option to try - believe me, I've been searching this one for nearly a year, but because my iPad synched and backed up fine I overlooked the simple task of trying the connection via the USB port on the back of the PC which is directly connected to the Motherboard.
Hope it works for you too - good luck.
I'm afraid the suggestions on that thread didn't help me.
Changing the folder caused my iPhone to make a new backup, but this backup still crashed about half way through.
I don't have any USB ports on the back of my computer as I am using a Macbook Pro, so that unfortunately won't help me !
My iPad syncs perfectly as well. Unsure of what it the cause of this.
Thank you for the suggestions though!
My case is iPad 4.
Deleted the Backup files in Library > Application Support > MobileSync as told above.
iPad = iPad 4 iOS 6.1
Machine = iMac 27"
iTunes = 11.0.1
Backup to This Computer & iCloud receive the same error message 'iTunes cound not backup iPad because the iPad disconnected.
For iPhone 5, it's OK to sync & backup in iTunes, but iPad 4 started this error message about 2 weeks ago.