my iMAC doesn't recognize a blank cd or dvd disk

Try this

When you insert a blank disc, your CDs & DVDs preferences

determine what happens next. If they are set to Ignore, then nothing

happens in the Finder.* Follow these steps to change the setting:

1. Choose Apple menu () > System Preferences, then click CDs & DVDs.
2. If you don’t have an optical drive built into or connected to your Mac, CDs & DVDs preferences aren’t available.
3. From the ”When you insert a blank CD” and ” When you insert a blank DVD” pop-up menus, choose any action other than Ignore, such as ”Ask what to do.”
4. Quit System Preferences.

There is a bug in Mojave. It doesn't display blank disks. You can still burn disks by creating a burn folder, adding files, right/control click on the folder and choose "Burn…" from the contextual menu.