If you have external or secondary hard drives connected to your Mac that you would like to backup with Backblaze, and are running macOS 10.15 Catalina, you will need to grant "bzbmenu" access to your removable volumes.
Typically macOS will display a dialog asking if you would like to give bzbmenu permission to read your external or secondary hard drives. If you see this dialog, simply click "Yes" or "Allow" and no further steps will be required.
If you accidentally click no, you can go in to your Security & Privacy settings, navigate to "Files and Folders", and make sure that the "Removable Volumes" checkbox is selected for bzbmenu.
Once selected, you will be able to continue backing up external hard drives!