Downgrading from Mojave to High Sierra ... lessons learned

[dnanian]

There's definitely nothing in that article that would suggest a special terminal command is necessary to recognize an external device at Option+Power On... I'm quite familiar with everything in there. Would really like to understand what, exactly, they asked you to do, since it's totally news to me.

With regard to formatting, yeah, welcome to the new Disk Utility. Two steps forward, etc.

You definitely shouldn't use APFS for a regular HDD that you're going to be running from as your regular startup drive. But, for 10.14 and later, it's fine for Fusion and it's fine under all versions for SSDs. It's also preferred for backup drives, since you're not running from them all teh time...

Anyway, SD maintains the "major format" when erasing. It won't change HFS+ to APFS or vice versa. But you can also erase yourself and then Smart Update, or whatever.