You want to be booted into Sierra 2015 Macintosh HD on your internal SSD with your external HDD connected when you run the command.
was pretty explicit.
I guess this is why I'm one of the only people in my entire large family who's not a teacher.
No! You boot into your internal drive - Macintosh HD - connect your external, launch Terminal - The only Terminal you can launch is the one on the partition into which you're booted. - and run the command, which will perform a "surface scan" on your external...not any specific partition, but the drive itself.
You DO NOT need to be booted into the external to run the command on it, because, as I tried to explain, it's a "subset" of MHD, or think of it as just another folder, the contents of which happen to be macOS installations rather than documents.
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
