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

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.

I deleted the .xml file but still got the error until I shut down both the Mac and the iPhone .

This error message popped up for the first time today, solved it by swapping the cable being used from a third party 'Anker' branded cable to a genuine bonafide Apple cable.

Thank you so much. This has been driving me crazy because the iPad didn't give this error, just the iPhone 5S.

Unlocking before synching did the trick. 🙂

I have had the same error 54 issue appear recently. Following the advice from experts here, I tried a few simple procedures -restarting, using the disk utility- to no avail, until thinking about some hints also here, I realised I had never used iBooks in my MacBook, just in my iPad, so I just logged in to iBooks in my MacBook, repeated the iPad sync-ing procedure and the issue disappeared!! The conclusion in my case is rather obvious, the logging-in procedure I describe seems to be a requirement for a correct sync, but what I do not understand is why this error only appeared for the last two weeks or so. The only difference I can think of is that I have been archiving quite a lot of PDFs in my iPad iBook Library recently. This is written in the hope that it may help others not very much computer-literate as I am, the same way that -quite by chance- it helped me.

Wow - I just unlocked my iPhone and synced again and no error! Thanks.

Thanks for your post, this solved the problem.

I had the exact same thing happen last night when I tried to sync new IPhone 6s with my Macbook. I was agonizing over this when I saw a suggestion to disable the iPhones pass code before attempting to sync. This worked. Try it and see.

You are right 100% Thank you =)

THANK YOU!!! So Easy!

I unchecked 'Encrypt iPhone backup' and it went right through with no error message!

...using macOS Sierra, iOS 10.2, iPhone 6s...by the way...