MacBook Pro stopped backing up to Toshiba external HDD
Help, my MacBook Pro has stopped backing up to my Toshiba external hard drive, having been working perfectly well up until 3 weeks ago.
When I attach the drive's USB cable to my Mac the blue light comes on on the HDD and the orange TOSHIBA EXT icon appears on my desktop. I can double-click on the icon and open all the folders and files in my previous backups. But when I try to run a new backup, the drop-down list sticks on "preparing backup" and stays there.
My equipment: MacBook Pro 13 inch, mid-2012. Processor 2.5Ghz Intel Core i5. 4GB memory. Running: OS X El Capitan version 10.11.6 (note: this problem did NOT coincide with upgrading to El Capitan, as seems to have happened to others).
Toshiba Canvio Basics external HDD, 500 GB (approximately 225 GB as yet unused), USB 3.0 connection.
MacBook Pro (13-inch Mid 2012), iOS 10