It's not a bug. The Drives toolbar isn't one of the built-in toolbars that you can factory reset. It's just a normal toolbar, and only really exists as a simple example which can be deleted if it isn't wanted (unlike the built-in toolbars which can be turned off but cannot be deleted to clean up the toolbar list).
You can find the original copy under C:\ProgramData\GPSoftware\Directory Opus\Buttons (put there by the installer) and copy it to your user config if you want to restore it.
The most straightforward way is to use the buttons that already exist on the default toolbars.
You can also copy or move them from the defaults if you want them somewhere else, which can be done via drag & drop, either from the toolbars themselves (if you haven't deleted those buttons for some reason) or from the Default Toolbars tab in the Customize window. They're also in the Commands tab.
Also: