Screen grab not working with external monitor connected

I've just got a new Macbook Pro, and I'm using it with the same external LCD screen as I was using with the old (2007) one. But, with the new laptop, I can't take screen grabs.

I've tried using Cmd-Shift-4 as usual, also the grab utility and the grab menu function in Preview. All I get is a bunch of diagonal shading, regardless of what was actually on the screen.

It all works fine if I disconnect the external monitor, or if I choose "Mirror Displays" in preferences. But, of course, both of those defeat the purpose of having a second screen, and it's really annoying to have to keep rearranging my windows after taking a screenshot.

With the older MBP, screen grabs work just fine. This is really bugging me, because I'm used to being about to Cmd-Shift-4 to grab a quick picture to collaborate on design issues.

Is anyone else having similar problems?

Could this be something to do with the new DRM on the DisplayPort?

Any suggestions would be gratefully received.

David

OK, this problem has vanished. The only thing I think it might be is that, when I check the colour profiles for each display, they're set to a combination I didn't try when I was trying to fix this issue ("Color LCD" for the laptop and "HW191D" for the external screen).

Anyway, it's working fine now, so I'm happy!