Update to Mojave results in NOT finding external CD player

Just to make sure that it is not being recognized, click on the Apple logo at the upper left of the desktop. Click on "About This Mac". Then click on "System Report". Click on "USB" in the left sidebar. See if your CD/DVD player is listed on the "USB Device Tree". If it is then you know it is being recognized. Also make sure it is plugged directly into the Mac and not a USB hub.

What Mojave version are you running?

What brand and model optical drive do you have?

How is it connected to your iMac?

Can we assume it was recognized with High Sierra?

Have you tried other ports and cables?

Have you tried it with commercial CDs and DVDs as well as disks you've burned yourself?

Boot with this Plugged In.

You mean an external CD/DVD drive, correct?

If so, shutdown your Mac, unplug the drive, wait for 15 seconds, plug it back in, and then reboot your Mac. Leaving it Plugged in, it should now be seen.