Unfortunately, there are so many other issues with v.16 and v.17 that I’ve had to move on to other ways to archive and burn discs, since Corel/Roxio seems to have lost touch with what is important to its users.
I was able to resolve the issue, but not without some serious effort and time dedication. I completely erased my root drive (SSD) and re-installed OSX 10.9 (yes, Mavericks). Then updated to Yosemite, El Capitan, then Sierra, checking to see if SD2F files would mount at each install (which they did).
I am now on Mojave (10.14.4) and the function to mount SD2F files works consistently. No idea what is or where the issue lies, but there is a MacOS systemic issue that causes this with Toast. Even though I had everything backed up, it was a long and difficult road to get my system back to where I wanted it. Proceed with caution, but in the end, this seems to work.