Thanks, Charlie, for your reply on the Bug I posted.
All the drives I used for backup have been formatted Ext3.
The first Seagate backup drive to give this error has checked out perfectly using Seatools and formats and saves Windows files perfectly, although the second, WD drive failed.
The third new Seagate drive in the original NexStar3 USB housing works fine. Weird.
Thank you for your help.