I found this explanation in a different discussion with a similar problem "stuck on preparing to copy":
"Problem: Itunes won't complete a sync with iphone / ipad, and is stuck on "preparing to copy items" indefinitely.
The fix is ridiculously fast and easy:
Go to your PHONE / IPAD, click settings, general, itunes wi-fi sync, click sync now.
Now go to itunes and sync, and it works as intended!
Hope this helps someone - like the people at Apple Support who had no clue how to resolve this."
Worked for me with preparing to sync issue and for several other users. Tried it again after the first working try and it seemed to work okay in iTunes without having to go back to the iPhone settings again.