Unity 5.3.3 Unable to render into cubemap, make sure it's marked as 'Readable'

I want to render custom cubemaps for some of my reflection probes. When I attempt to run the Wizard (http://docs.unity3d.com/ScriptReference/Camera.RenderToCubemap.html) and place an existing cubemap into the slot and click Render, I get the error:
Unable to render into cubemap, make sure it's marked as 'Readable', and it fails

How so I "make sure it's marked as 'Readable'"? I am on Windows 7 Unity 5.3.3 pro

I duplicated an existing cubemap, so it would not be "in use", but I still get the error.

Creating a new cubemap (in Wizard) doesn't fail, but I need to save it to be able to reuse it.
Do I have to create a new cubemap and save it? How do I save it (in c#)?

Seems like a configuration issue, but how do I fix this?

BTW: my files are set for full access

Thanks,
Doug

 

I was able to save my newly created Cubemap via c#. A bonus is I am able to set the readable flag on this one, so now I can use this one without causing the error that necessitated its creation. Irony. It appears that the format changed to Legacy Cubemap. In my newly created cubemap, I can see all six images:



Compared to using Unity generated cubemaps from existing reflection probes:



Now I just assign this cubemap to my reflection probe and I have what I want.