I have a Macbook Pro running Sierra with three USB external hard drives. I have my iTunes library one one of them. This one I formated in NTFS because I thought I would carry it around sometimes, turns out it's always home. No problems until last week when I copied a few mp3 files onto its root folder and dragged them into iTunes (a folder within that hard drive), I got "Attempting to copy to the disk (name) failed. An unknow error occurred (-1401). If I copy those files to another external HDD then to iTunes, same outcome. But if I drag them from the Macintosh HDD, they copy fine. The main concern now is that some mp3 files are gone from the iPhone after syncing, greyed out and when trying to play them "Item Not Available. This item can't be played". Those missing tracks show with dotted circles when viewing iPhone contents on iTunes. Lastly, when I sync, those tracks will not copy over and final message is "iTunes could not copy (name of any of the troubled tracks) to the iPhone because an unknown error occurred (-1401). I ran a Repair on Disk Utility and it went fine, though I read DU won't actually do anything on an NTFS formatted drive. I read about -1401 and some mention permissions issues. I can't edit permissions via Get Info because it is NTFS, so the lock to edit doesn't show up. The external HDD drive's sharing & permissions show up as "custom access". I have a backup of this HDD drive on a Mac OS Extended (Journaled) drive, this includes my entire iTunes library, I use a daily automated task on Carbon Copy Cloner to this end. So the only thing I can think of is to erase the HDD drive containing my iTunes library, reformat it from NTFS to Mac OS Extended (Master Boot Record), copy all contents from the backup onto it, and hope that this error will be fixed. Can you think of prior steps to take doing this? Thanks for reading and I apologize for the long message, wanted to make sure I gave as much info as possible.