My thinking is that this is because the latter was backed up using WD proprietary software that Acronis cannot/won't use (a bit like a German speaker refusing to accept a Spanish book in his library). It seems that the backups done using the WD software did not backup.
When trying to do so, I discovered that only a very small portion of the files were transferred, these being the odd backups I had performed manually.
I now want to clean up the My Book Elite and use it as a secondary backup source but first need to transfer the existing documents onto the new drive using the Acronis backup facility. I now have a 4 TB WD Passport drive and am using Acronis True Image 2019 for this purpose. Over the years I used the WD Discovery backup software that came with my WD My Book Elite to back up my desktop.
