Error Message: Attempting to copy to the disk "Macintosh HD" failed.

After successfully backing up my iPad Mini 4 to iTunes via Wifi this message appears:

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

Is this anything to be concerned about? I do not notice any problems with either the iMac or iPad.

Hardware:

iMac Late 2013, OS 10.11.1

iPad Mini 4 iOS 9.1