Attempt to copy to disk failed. unknown error (-54). Why?

Beginning last week, I have been unable to backup my iPhone 7 Plus to my MacBook Pro in iTunes. The backup and syncing seems to go through the whole process then, at the end, a window pops up saying "Attempting to copy to disk Macintosh HD failed. unknown error (-54). I have already tried deleting the most recent backup, in iTunes Prefs, but that did not solve the problem.

Thanks in advance for your help.