Yes you can have too many backups and get confused; which one is the latest, has one got stuff that another doesn’t, etc …
Yes HD units do fail but not all at once, unless your house is burnt down of course but then I think my music collection would not be my highest priority and I do have all my CDs stored in the garage. Don’t be too obsessed, yes you need a backup but don’t go OTT, after all how Is your vinyl collection backed up?
You really should create a proper backup regime and stick to it rather than just making copies whenever the mood takes you. Backups are not just to restore your music when an HD fails but also to restore it if some major mistake has been made and the music collection has been compromised because of some editing.
Use the HDX incremental backup to your NAS and then you can make another copy of this backup file to some safe external media if you wish.
The incremental backup doesn’t appear to backup the downloads folder on the unit so some other backup regime should be put in place for this; I haven’t done this at the moment but am just about to create some windows batch files to do this for me.
Allan