Baking multiple object's AO in 3ds Max: I keep getting black BGs
Hey guys, I'm learning how to bake texture maps at the moment. So in max 2012, when I hit 0 for render to texture and select several different meshes, add an Ambient Occlusion output and hit render, the first map always comes out right (with a white BG). Subsequent maps come out with black BGs and are harder to use. This doesn't happen when I do it seperately for each mesh. Why is this? Also, is there any way to render a single image output for multiple meshes? It'd help me skip a step compiling them in Photoshop. Thanks for your time 
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As for the black meshes, that could be a lot of things. Might want to search for black RTT meshes on the forum, I know it's definitely been discussed before.
Could be a lot of things. Corrupt mesh, in need of reset xform, etc. You can try resetting xform on the problem meshes. You can also make a new plane or box or whatever, then convert that to epoly and attach the problem mesh to it, then delete the box out. Can also try merging all content into a new max file. Finally, exporting the whole scene out as 3DS or OBJ and reimporting.
its usually fixable in photoshop by selecting according to alpha, copying to a new layer and then defringing/dematteing over a plain white layer but its a toss up as to whether that takes longer than making the bakes individually
if they all share uvs then i attach everything together and bake off that
http://cg.tutsplus.com/tutorials/autodesk-3d-studio-max/how-to-bake-a-flawless-normal-map-in-3ds-max/
But i would advice you try simple baking first than move to multiple bakes at same time. Normal and AO is a big subject with too many 3rd party scripts and apps to do this process alone.
here check Poly wiki before asking ... helps alot.
http://wiki.polycount.com/NormalMap
Hope it helps
he's getting a black background colour on AO bakes when he is baking multiple objects despite setting the background colour to something else (ill assume white). I've seen this under max2012 as well so its highly unlikely its the fault of anything in the meshes or technique (8 years as a tech artist ought to put me in good standing technically speaking)
I also suggest baking it out in one mesh format.
You have to be real lucky to get AO to baked out properly in Max, I suggest xNormal or Blender for free. If not, and you still insist in Max, VRay is your only other solution for Max.
I think Max 2009 is the only one that didn't have these problem (apart from streaking, but a simple dilate of your layers is all you need at that point in PS).