I have 3 USB external disks that have been working perfectly well on OS X 10.11 but this problem started as soon as I updated to Sierra.
I have tried everything possible that has been suggested on various forums but nothing has worked.
I last had this problem with os X 10.9 (I think) which was fixed by one of the updates. Now it is back!
Please help. It is driving me mad!
Edited by shirland (Wed 02-Nov-16 17:52:02)