Q: iphone backup fails with "required folder not found"

Basics:

OS X 10.11.4. I

iPhone 6+ ios 9.3.1

iTunes 12.3.3.17

Photos 1.5

I keep getting the following message after I plug my iPhone into my iMac (2010):

"Attempting to copy to the disk 'Macintosh HD' failed. The required folder cannot be found."

The backup process appears to hang on "Step 7: Preparing to copy items."

I have not moved my Photos or Music libraries from their default locations.

I see a lot of people reporting this problem, but no solutions.