Well, OK. I tried it, and got nowhere. There may be ways around the problem that I am not aware of, but it appears that, once I formatted the SSD to APFS, there was no way to down-format to EFS+. I presume that the fact that I can't down-format from an APFS disk (or SSD) indicates that the controlling volume is, in fact, formatted as APFS. I don't know any sure way to inquire of a disk how it is formatted, although there may be one. Like you, I am working in 10.13.6, on a Mac Pro, so I want to set up one drive with Mojave, which I've done now. But I still have all of my backup files on HFS+ drives, which Migration Assistant refuses to touch. At least, the High Sierra Mig. Asst. refused. I shouldn't include Mojave in that. I'll finish and try it from Mojave. Who knows?