how to find locked files for unknown file error (-54)

As I attempt to synch my iPhone 6 Plus in iTunes, I receive the error "Attempting to copy to the disk Macintosh HD failed. An unknown error occurred (-54)." I have done some research across the discussion communities and found a few articles related to locked files.

Steps attempted:

1. I have rebooted my Mac mini - no change
2. I have used Disk Utility to verify disk permissions on the main boot drive "Macintosh HD" - no change
3. now I'm trying to use terminal to find locked files within my iTunes library folder, as i have 9,948 song files in the library.