[Don_S]

If I use a box like the one in the picture and use a metal conduit nipple to connect it to a larger metal box do I have to run a ground wire from the small box to the larger box? A switch will be installed in the smaller box and connected to the ground screw. The larger box is grounded but I can't get to the grounding buss to add another wire.

This box is die-cast.

[pansixt]

Don, This is the best picture I could find to illustrate the grounding bond between outlets or junctions. If you're putting a switch here, the principle would be the same.
Edit for a second pic with switch. So the answer is no it is not necessary to run a ground from box to box since you will have a common ground.

[richidoo]

I'm not licensed electrician, but my gut tells me Yes, you do need copper ground wire between the ground lugs on junction boxes, because the conduit connections are intended only for mechanical protection of wires from physical damage, and the conduit joints and connections won't necessarily provide good low impedance ground like copper wire. So without a copper ground wire, a fault in the new switch box may not be sensed by the ground wire or breaker in the big box.

Maybe you can ground the new box via another nearby grounded fixture, box or a ground wire nut nearby. Hooking into a copper ground wire that comes from the inaccessible ground buss.

EDIT:
I was wrong...
https://www.jlconline.com/how-to/electrical/grounding-electrical-boxes_o
According to this electrician, metal conduit is acceptable ground between boxes. Sorry!

[Speedskater]

Ridge EMT metal conduit is a Safety Ground (EGC) conductor. But it has to be continuous al the way back to the main breaker box or connected to a Green Safety Ground wire.
Flexible metal conduit is more complicated. Some are Safety Grounds and some aren't.

[Don_S]

Thanks everyone. I remembered I had a email contact for a retired electrician. He said the metal conduit would be a suitable ground between the boxes.