I was able to solve the problem as follows:
Ran fsck (e2fsck) on the Backup Drive and it showed nothing, but using Disk Utility, SMART Data revealed errors with the Sector Count.
Then ran e2fsck with the -c switch to check and map badblocks
e2fsk said there were inodes with multiply-claimed blocks
for each file with a bad blocks inode, it paused and asked the following two questions which I answered no and then yes, to allow it to delete the "problem" files on the backup drive.
Clone multiply-claimed blocks<y>? no
Delete file<y>? yes
Finally, reran rdiff-backup, but added the --force switch, which caused it to replace the deleted backup files with copies from the original Data Drive.
rdiff-backup --force //DataDrive/Path /BackupDrive/Path
Next step will be to get a new backup drive.