Why is the Surface Scan unavailable for my internal drive?

Beginning in macOS 10.13 "High Sierra," certain low-level operations became unavailable to applications like Techtool Pro. One of these operations was reading the raw disk for the physical drive containing the startup volume. Because of this, we cannot do a surface scan of a drive if it is currently being used as a startup disk. In order to solve this problem, you will need to start up from a separate physical disk in order to run the Surface Scan test on your main startup disk. This can be accomplished by building a Protogo device on a separate disk, such as a thumb drive, or building your eDrive on an external disk.