It is worth noting that Apple, Micromat (TechTool Pro), and Prosoftengineering (Drive Genius) have all removed surface scans from their applications (granted Micromat and Prosoftengineering followed Apple's not too subtle lead) because in the opinion of their engineering staffs surface scans of SSDs could result in reducing the usable lifespan of those drives.

In the case of a Fusion drive the idea of scanning only the rotating rust segment is intuitively appealing, but the fusion takes place at a very low level within the logical structure of the volume and might require breaking that fusion to access only the rotating rust portion of the drive. In my experience it is possible to separate the two drive segments, but the process is destructive so all the data on the fused drive is lost. The process would look something like…
  1. clone the fusion drive to another disk
  2. boot from the cloned drive
  3. break the fusion
  4. perform the surface scan of the now blank HD portion of the former fusion drive using whatever tool you desire
  5. re-fuse the SSD and HD portions
  6. Clone the data back to the new fusion drive
  7. over the next several weeks or months MacOS will re-optimize what is stored on the SSD and what is stored on the HD.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein