Currently, it seems very difficult, but feasible improve in Mojave, but not directly.
Currently, it seems very, very difficult, but feasible update (in) Mojave.
Similar to avoiding the APFS conversion on the High Sierra upgrade, but this time need to go the road of the clone.
But there is some catch to that.
Mojave will boot from HFS +, once installed on it.
The installation program will convert the drive on which it is installed, regardless of what happens. The command-line option to ignore the conversion has disappeared. Apple really wants to impose the adoption of APFS.
This means that you must install Mojave on a separate volume, which will then be in APFS format. Then you need to reformat your internal main disk which is the real Mojave target as HFSplus. Now clone the external volume on the target.
Restart in Mojave – and keep the external player as a mullet.
As it is currently the real catch:
App store app (fromerly Software Update) will currently does not work when Mojave is started from HFSplus. It will launch and update properly when running on APFS.
This means that, on system updates, you must restart your APFS mule, update to that location, and then reclon the updated Mojave file to your internal working disk (perhaps intelligently excluding it from folder / Users?).
It is unclear what else could break by this method. It seems to work "well" in terms of functionality, and it seems to work even better thanks to HFSplus.
But the procedure is more than cumbersome.
Apple could do two things in the future:
- Disable startup completely from HFS +.
- Enable update on HFS +
If you care about him, like me, write to Apple to ask for this feature. Fast, urgent, it is mandatory.