Can anyone tell me how to eject a disc from a viewer device CONNECTED to my iMac?

1. Open System Settings app
2. In the search bar type "CD & DVD" and click on the result.
3. Where it says "Show eject in menu bar" select that.
4. Click on the new menu bar icon in the upper right of the display (it looks like an up arrow with a line under it) and select the option you want.

Another option is to enter this command in the Terminal app followed by the Return or Enter key:

drutil tray eject

However, the best way is to make sure this checkbox is checked in the Finder's general settings pane:

This lets the disc show on the Desktop where you can drag it to the Trash bin to eject.