Something in High Sierra broke this completely and while Apple has not fixed this yet, Arthur Gareginyan over at posted a fix back in October. Up until High Sierra, all you had to do was launch Disk Utility and partition the external disk as HFS+ Journaled. I've recently been approached by a number of folks who like using portable hard drives for travel and for their photo libraries for their laptops who have been unable to prepare the drives with an Apple file system and partition.
