Elememts 2018 fails backup

I've never had the backup work reliably for long periods. It will randomly fail on folders.  Now it reaches about 70% a givers error writing files.  I can mobe the effected folder and it will reach 71% and stop on a different file. I am backing up to an external drive that otherwise works fine.  

I have reformatted the drive, created separate partitions etc.   

I have optimized the catalog and it works fine. I do not see the option to repair the catalog.  

How can I troubleshoot further?