Symmetry in Sculpt Mode... is this right?


Just started getting my teeth into the sculpt mode and I’m a bit stumped on matters of mirrors.

If I chop a box half down the X axis and go into sculpt mode, is there a way to show the mirror of that half?

If I have a whole box, I click Symmetry>Mirror X and the sculpting affects to both sides, cool.

But if I’m modelling half a box in edit mode with the Mirror modifier (which won’t work in sculpt mode) is there a way to see the absent half in sculpt mode.

I have done some Googles, but despite some videos appearing to show otherwise, clicking Symmetry>Mirror X does nothing when I’m starting with half a box - Is that just the way it works, or am I missing something obvious?