Forget about finding backup location.
just go to iTunes preference
Delete backup and make a backup again
I hope it works...
So I'm hopeful I've solved this for my situation. Backing up over USB always worked on my iPad but not my iPhone, so it had to be something with the iPhone and not the cable. I've tried all suggestions mentioned here without success. It dawned on me that I have wifi sync enabled on my iPhone but not my iPad, so I turned that off and I've been able to back up my iPhone three times in a row now without incident. That hasn't happened in a very very long time...
If any else can confirm this fixes the issue then I'll make a bug report to Apple.