MacBook Pro 8,1 as below. It has a new WD SSD I have two 4TB "back ups" partitioned with 1T each to clone the IntHD. This I do with SuperDuper! Those old ones are Seagate.
Works perfectly. Boot holding down "Option," type in Firmware Password, pick clone. Great for testing new updated bloatware! Also for fixing mistakes--"do I really need this file in my System?"
By coincidence one of the 4TB Ex-HD started to come back "NOT SMART!!1!!" Blah . . . blah . . . need more room anyways, so I got, on recommendations, a Western Digital "My Book" with 8TB to accommodate all of that refined but highly questionable French lichen porn.
Potential Problem the First! I formatted it as APFS. Why? Because I did that with my nearly 5-year-old 4TB Ex-HD when I updated my older Int-HD . . . yadda . . . yadda . . . it worked! Perhaps WD "My Book" does not like it since, after searching, all of the instructions deal with formatting it to the older Mac architecture.
Potential Problem the Second: Booted to "Me" on my Int-HD, I can use both partitions. Hook it up and they load immediately. I can watch Jean Paul and a Rock while loading an application from the clone. I can choose it as the "Startup Disk."
If you do that, you get the Flashing Question Mark [of Doom.--Ed.]
Or, if you boot and hold "Option" you cannot see and choose the WD "My Book" clone partition. You can the older clones.
After Searching: I read a lot from, say, HERE about the possibility of "Option ROM." I tried all of those solutions listed . . . FAIL! Holding Shift-Option-Command-Period does nothing. I even tried resetting the PRAM--remember doing that?!--based on some other suggestions . . . as well as changing USB ports. This is a problem, I suspect, with WD since if you boot while pressing Option, you can add on the Older Seagate 4TB and it will, in a few seconds, show up as a boot option.
I have queried WD Support to see if there is "something" I am not doing--like, maybe, it only works with the older architecture, but the nearly five year-old Seagate has no problem with APFS. They take a few days to respond; if they give a solution I will post it. If I have to return the damn thing, that would result in a sudden inhalation whilst something inhabits the oral cavity.
"Y U NO YUSE TIME MACHINE!!11!ELEVENTY" LONG ago, TM would not make a bootable clone, so I went with SuperDuper! I believe that has changed, but I am a creature of habit. I am willing to try that if that is a known solution.