Attempting to copy to the disk Macintosh HD failed. An unknown error occurred (-54).

This helped me too! I'm using windows 7 and when I checked iTunes folder security permission, there's wmpnetworksvc in user list. Removing that from the list, and the error went away! I've been trying to solve this problem since a week ago and nothing's helped, thank you so much!

For me the problem was related to syncing Books. None of the above ideas fixed the (-54) for me, but when I turned off "Sync Books" in iTunes, the error went away. So I went into the iBooks application, deleted the most recent book I had added, and then turned "Sync Books" back on again in iTunes. Syncing now works, including syncing of books, with no (-54).

Some more details: I noticed that after deleting the problematic book from iBooks, it reappeared again there for some reason. I must have somehow gotten two copies of that particular book added to my iBooks library, but the second copy wasn't visible in either iBooks or iTunes, and manually deleting the book from within iBooks must have deleted the bad entry, leaving just the good one and thus fixing everything, maybe?

(I also tried the "ls -lOR | grep uchg" trick mentioned in another thread, but it found no locked files anywhere in my user folder, including the iTunes subfolders, so for me locked files seems not to have been the problem. Note that the other thread says to use a lower case "o" flag in the "ls" command, but according to the ls doc and my testing, it should actually be upper case "O".)

I really wish Apple would make this error message more specific, at least specifying which file triggered the "unknown" error.

Hopefully this info might help someone.

I had exactly the same problem here. By unlocking my phone, connecting to iTunes and then syncing the problem has gone away.

Similar issue; also books, though in my case it was PDF files I had saved to iBooks from an email on my iPad. I suspect a permissions issue of some kind on the files. Once I noted that the PDFs in question (payslips I had emailed to myself from work) weren't in the iBooks app on my Mac I deleted them from the iPad and the error went away.

Interestingly everything else would sync fine, it's just that the sync operation wouldn't finish, so clearly there's some kind of bug in iTunes causing this. In the past I've noted space issues on my iOS devices from deleting things manually on the devices after they've been put there using iTunes. It seems to not like that, so I've tried to keep in the habit of only deleting media using iTunes if I installed it from iTunes via sync.

I just had the same error problem twice in a row - the second time iTunes crashed. I unlocked the phone and started again and watched the screen for any sleeping and touched the screen when it greyed - the backup and sync worked okay.. So thanks for the simple suggestion - I hope it isn't going to have to be a regular procedure.. BTW I had just updated today to latest iOS 9.2.1 and computer OS 10.11.3 so looks like more tinkering required by Apple. The systems are getting more and more complex. Everytime I try to Sync it asks if I want to transfer updated apps - I agree but then when I look at the apps held in iTunes on the computer - they all need updating again - so unsure why that happens.

Yeah that doesn't make a lot of sense. I think it only transfers new content off the device and unlike iOS devices, itunes doesn't appear to have an auto-update option so I've just accepted the fact that I need to manually update stored iOS apps. I don't auto-update apps on my iPad so it suits me to read the update descriptions in iTunes before selecting the update option.

I have started keeping the apps up to date in both the devices and iTunes because last year I had several issues with my mini iPad not completing a backup and sync and I think it even had the message in the topic Attempting to copy to the disk Macintosh HD failed. An unknown error occurred (-54) on some occasions. Once I made sure the apps were up to date in iTunes separately from the device apps, the problem seemed to disappear and the sync would complete so I figured syncing meant that each storage would have the most up to date version of everything and had to match for iTunes and the iDevice to be happy.

I think there is the assumption at Apple that iCloud will be used for syncing and iTunes is not so relevant any more. On a call to AppleCare it was suggested I open up iCloud syncing but when I said the time to sync was something like 36 hours, the Apple Helper realised it wasn't a good option... When you live in Australia with the abysmal internet speeds - often ADSL2 runs at 1MBps instead of the originally marketed 20MBps.. and upload speeds are often 250kbps so iCloud syncing is not easy task for anything that involves sizeable uploads.

Even with fibre broadband I prefer the reliability of a wired connection for syncing; plus my content is a mix of stuff I've ripped and iTunes purchases. Best I could reasonably do is a wireless sync to my Mac and I did experiment with that, but that wasn't consistent and at least with the wired connection if the sync hangs you can just pull the plug to abort it.

I was having a similar issue. The error: "Attempting to copy to the disk “[external FW800 RAID]” failed.

An unknown error occurred (-54)."

This was coming up every time I plugged my iPhone 6s running 9.2.x in to my iMac running 10.11.x with iTunes 12.3.2.35. My iPhone was set to back up to my computer and this is what I believe was causing the error. Then it would copy some new album art or a podcast or something and then iTunes and iPhone would never stop syncing. It just said something like 'waiting for items to copy from iPhone' and I would have to quit iTunes to get them to stop.

I tried many of the suggestions above, though as noted you can no longer repair permissions in El Capitan, to no avail.
After seeing this post it dawned on me that I hadn't started with the simplest thing first and I assumed it was an issue on the iMac and specifically the disk. I rebooted the iPhone and it worked like a charm.

Sometimes it really can be that easy........sometimes.

Thanks! this worked for me. I tried everything, ensuring i am on latest version of iTunes+iOS, checking and repairing permissions, restarting mac, different cable, etc etc. but just simply restarting device worked at the end.

I was having the same problem with my iPhone 6s+ and iTunes after the iOS 9.2.1 Update. Deleting the XML iTunes library didn't work and I wasn't comfortable resetting permissions due to the way my work applications are setup. What did however fix the problem is simply disabling my Avast Anti-Virus file / web / mail shields. Considering such, I'm guessing iOS 9.2.1 introduces incompatibilities with anti-virus software, so try temporarily disabling any during sync.

I had the same error when trying to back up my 6S in iTunes on OS X El Capitan. I tried many of the things mentioned (reboot, reboot in safe mode, restart phone, etc.) and nothing worked. The solution for me was to unlock my phone before backing up. (I should have read the full thread before trying any solutions...) I'm not sure why were getting an error message about the Macintosh HD when the problem is that the phone is locked...

mikebdba wrote:

I had the same error when trying to back up my 6S in iTunes on OS X El Capitan. I tried many of the things mentioned (reboot, reboot in safe mode, restart phone, etc.) and nothing worked. The solution for me was to unlock my phone before backing up. (I should have read the full thread before trying any solutions...) I'm not sure why were getting an error message about the Macintosh HD when the problem is that the phone is locked...

THANK YOU! Unlocking my 6 Plus before syncing is the one thing that finally worked for me!

Aismaiil wrote:

Unlocking the phone before syncing solved it for me!

OMG this solved things for me. So simple. Thank you!!

Apple needs to fix this!!

Yup, all I had to do was unlock my iPhone 6!

A more specific error telling us to unlock the attached iPhone would be convenient.