Solution below copied from elsewhere.
I have verified that this resolves the issue under MacOS 10.11.6 (El Capitan). I had Toast 11, 12, 13, 14, 16, and 17 installed. Now just 17 *with* 'Toast It & Mount It' (apparently essential to resolve prior inability to mount Sd2f and Toast disc images).
Thank you for contacting Corel Technical Support.
If the procedure on my previous email did not work, please try to do the following.
Quit Toast, trash the entire Toast 14 Titanium folder (located in Finder > Applications), then do the same for any previous versions of Toast, if you still have any installed. You need to remove all previous versions of Toast to avoid possible conflicts. Next is to check for the files listed below. If you see any of them, trash them:
"Roxio Toast Prefs"
You should be able to locate these files in ~/Library/Preferences. On the taskbar at the top, click on "Go", and select "Go To Folder...". Type in "~/Library/Preferences", without the quotes. Locate the above files in the Preferences folder that comes up and delete them. You must use this procedure because this particular Library folder is hidden from view, and this is how to open hidden folders.
Once the procedure is complete, reboot your Mac, then reinstall your Toast 14 Titanium. Install any available updates.
Let me know how it goes.
Please do not hesitate to contact us with any further questions.
Corel Technical Support Services
I discovered that you must also install the "Toast It and Mount It" extra or you won't be able to mount disc images in Toast. Remember that when you do the install/register.