Acronis TrueImage 2014 trashes target drive

The first three times this happened I thought I might just be my bad luck, but I am now almost sure the fault is Acronis.

On a regular basis I use a 2014 boot CD to backup my systems to an external USB hard disk.  Every month I backup several computers to the external target drive.  On occasion I need to restore one of the images.  Everything works fine until...

When making a backup, the Acronis CD boots okay, I start the backup.  I can see the source hard disk okay.  I can see the target USB external hard disk okay, including the names of older backups.  Then when I try to enter the name for the new backup Acronis responds that the file name I entered is not valid.  From that moment on the target disk is trashed.

The target disk can no longer be read by the Acronis boot CD, cannot be read by Windows and cannot be read by Knoppix.  The target disk appears to be unformatted.  When the disk is reformatted, it works fine until the next time Acronis trashes it.  This has now happened with two different target hard disks, so it cannot be a problem with the target drive.

Every attempt to recover the data on the trashed drive has failed.  Fortunately the only thing I have lost is backup images, but I now have less faith in Acronis than before.