What are all these extra files?

I’ve got a 2TB My Book, serial [Deleted-Privacy], Product ID 1130, Firmware revision 1016. when I look in the email backup files in the .swstor directory (AppData/Roaming/Thunderbird/Profiles/nnnnn.default/Mail/pop3.netzero.com I see a whole pile fo 2Gb “files” that appear to be generated about every two hours. I keep wondering why my drives fill themselves up, and this appears to be the problem. Can I just delete them? How do I keep the WD SmartWare from generating these files?

If you set Smartware to backup these folders, then you may exclude them from the backup sources.
This is part of the Files Backup plan. If your actual setup is using the category backup, you nedd to switch to Files backup to select individual folders / folder trees to be part of the backup.

These files are not in my backup source. WD Smartware is generating them, and filling up the backup drive with them…

I don’t think so, the path in your file explorer shows the content of your Thunderbird mail box for the account related to netzero, the path is %appdata%\Thunderbird