I have recently ordered a new Mac mini with the idea of putting the Mac Pro into semi-retirement and I decided to order three new external drives for mini.
There will be a 1TB SSD for general data, a 1TB USB 3 for SuperDuper and a 2TB USB 3 for Time Machine.
I first tried the SSD in disk utility and found no option in Mojave to format as APFS, so I tried on my daughter's MBP running Catalina and there it appeared as an option by right clicking on the drive with a warning that the the drive would not be able to run on an older system, so I left it for the moment. The same option in Mojave was greyed out.
I have yet to look at the drive destined for Time Machine, which will remain as Mac OS extended, however I have the same issue in Mojave of not being able to erase and convert this to APFS for SuperDuper. I have followed the thread at
https://www.shirt-pocket.com/forums/...ead.php?t=5738, however it does not seem to work that way.
The mini is still being shipped, so no huge rush on this but I am a little perplexed, because I had no difficulty before converting drives to APFS, whether internal or external.