Weird. You must have had a dud. Couldn't possibly have been user error.
I'm sure I did get a dud. Teach me to buy a first run of a new product! (Ooops, i did that again w d800s and XT1, no issues tho except the fuji's light leak)
The 5d2 banding flaw I first noticed in gray midtones and clear blue sky, then discovered I was not the only one complaining about it.
There were 2 sources of vertical banding. One was a FPN read noise with an 8 pixel cycle, like the 7D/60D/etc.
The other was a different issue, and actually the first flaw I'd noticed in the 5D2's smooth midtones. After looking more closely I found the other noise issues. Then I was just seriously annoyed!
But, back to the other banding issue. I've also just discovered it in one of my recently acquired 2nd-hand Fuji XE1s.
About the central third of the sensor is affected. When doing a dust-check at 200mm and f/22, I found this central region was also inhabited by wide bands of slightly lighter-darker areas, but not perfectly regular in alignment. When I shot another test w-o any lens, I could no longer see this issue. So it is some strange sort of interference effect?.. Anyone have a good guess?.. I thot it if was a simple interference it might make radial patterns but it created wider, relatively vertical bands.
When I still had the 5d2 I didn't try a no-lens shot to see if that effect was still there.
If I get some time, I'll post an example from the Fuji, it's very similar.
I suspect it might be minor misalignment of micro-lenses over their pixels producing subtle variations in effective QE.