How to debug USB storage issues on a MacBook?
In the past using Macosx El Capitan, I would sometimes see
the message USBMSC Identifier (non-unique) in dmesg
after I had disconnected my external drive and tried to 
reconnect it later unsuccessfully.

My blackbox perspective of what was happening is that upon
the attempt to reconnect, the already cached mac id of the
usb device was recognized as already existing, but interpreted
as a different device, so the "reconnect" failed.
In that case, I disconnected the external drive from USB again,
powered down both the drive and the computer, waited a short while,
then reconnected the drive and booted.  The id was found as 
unique again and the mount of the external drive succeeded.

I see the same log message upon another error in 
Sierra 0.12.6, however. I'm seeing the USBMSC Identifier (non-unique
log message when I'm not able to mount an external drive which 
has the same vendor ID as another previously connected external
drive, but different complete mac ids.
I haven't been successful at getting the OS to recognize the
second drive as unique yet.
I haven't tried to mount the drive using another OS yet, but thought
the "reset" scenario above might be helpful.