I’d like to mount an articulating arm for my 43” TV in my basement, however have metal studs unfortunately. The mount is made for a single stud install. Luckily I have access to the back side of the wall, and could work from there. Could I install a wood stud next to the metal stud and mount that way? Or would a large piece of plywood between the studs behind the drywall be a better approach with some toggle clamps holding it in place? I don’t really want plywood on the outside of the wall if possible. Thanks for the help!
An articulating arm swings away from the wall and this creates different forces than just attaching directly to the wall. You didn't note the distance that it can articulate nor the weight of the TV nor the vertical distance between the bolts of the bracket to the wall and all are factors that determine the 'moment' and therefore the force of the tension / compression affect at the wall (consider using your hands to hold a piece of plywood up vertically by the edges, you'd have to push out at the bottom and pull in at the top, as well as lift up the weight of the plywood to keep it in place).
For metal studs the critical element is most likely the flange or face that the dry wall and bracket are attached to, it will bend out of plane, in and out of the wall, likely before the stud as a whole would fail. What resists that is the thickness of the metal. The force required to bend the flange likely isn't high. Metal stud design is COMPLECTED and there are MANY different types (some are very flimsy) and of those the least expensive is chosen, that is, the one with just enough strength to do the purpose.
With that in consideration, and that you have access to the back of the wall then I suggest attaching directly to new full height wood studs. You can easily locate the studs by first determine where the bracket needs to be located based on where you want your TV, drill holes through the dry wall as if you were going to attach it, but don't yet of course. On the back side, use those holes to located the new studs.
Strong and easy peasy.
There are two solutions, both of which you've outlined:
1) Brace two studs with 2x4s and mount the tv to that.
This is hit or miss -- stud spacing has to match the bracket spacing, and you need to open the wall up enough to add the new studs and ensure that they are stable and that the metal studs won't buckle.
2) Put a large piece of thick plywood across multiple studs and anchor to that. This allows you to have different spacing on your studs vs your bracket. It also creates a more rigid platform.
I elected to do #2 -- since the plywood is significantly smaller than the TV you can't see it anyway.
Not sure how you'd get the plywood behind the wall -- you could remove the sheetrock and replace it with plywood, but you probably need thicker plywood than the standard 1/2" sheetrock, so it will stick out anyway.