I’m one of those few people who still syncs my iPhone with iTunes to maintain a local backup. Syncing with my computer rather than using the Internet to move data also improves transfer speed, especially when moving large items (movies, music, apps, etc) to my pocket.
Usually, everything syncs smoothly. This past month, however, an odd issue cropped up. Right after completing the first step of the sync, which is backing up the device, iTunes would throw an error: “Attempting to copy to the disk “Macintosh HD” failed. An unknown error occurred (-54).” After clicking OK, the sync appeared to continue as normal.
This went on for a while, until I got disgusted with seeing this error every time I synced my phone. Then I decided to do something about it. Online research, which plays a vital part in my troubleshooting process, did not reveal any key insights. So I was left with nothing but a hunch: in passing, I noticed a PDF, which I had saved on my iPhone from Mail to the Books app, was not present on my Mac when I went looking for it. I knew books were supposed to be syncing. Could this be the cause?
I plugged my iPhone into my computer and found the device in iTunes. Clicking Books in the sidebar, I noted that “Sync Books” was checked off, as expected. I unchecked the box and synced my device. To my surprise, this sync completed without any error! Checking the box and attempting another sync caused the error to recur.
To get to the bottom of the problem, I un-synced all books once more, and copied the offending PDF to my computer before deleting it from my iPhone. Back in iTunes, I re-synced this PDF and all the other books to my iPhone once more. And to this day, syncing goes without a hitch … until I save a PDF to Books on my phone. At least I know how to resolve this issue now.
Apple Assistant 