At this point this seems to be an issue with the backup tool. Visit Microsoft web site for support on the application, If the drive is being recognized and you can copy and paste on it you should be able to backup on it also using any backup software, I would go online and find another Backup software and test the drive with it.
Paragon and Easuse have free backup software. It also depends on what you are backing up. Just the data or making a system image. If it’t just data then a sync program might wotk better.
Actually, i have using easeus todo backup free to backup my computer. not only the files, it also can backup the system hard disk to the new hardware. i think this free version is the most versatility software on the market. it offers many functions that just can be found in the paied tool. what more it is free for both business and home users.