Originally Posted By: freelance
I don't know if you can use Disk Utility from an external drive to change the file system of the internal drive, but if you can, then you could use DiskWarrior on the interal drive, too. That's what I did on my 10.13.6 set up.

I just tried to reformat the internal SSD on my 2019 iMac running Mojave. Disk Utility would not allow anything other than APFS, so we'll just have to wait for DiskWarrior to catch up.

iMac (19,1, 3.1 GHz i5, 12.7.4, 40 Gb RAM); MacBook Air (1.8 Ghz, 8 Gb RAM, 10.14.6, 256 Gb SSD) Vodafone router and Devolo Wi-Fi Extender, Canon TS8351 printer/scanner.