So this is weird. I've used dual monitors with my exact setup before. but now when I click on the "Preview to an External Monitor" button, it brings up the"Preview Device" options instead. No biggie, right? I just have to choose which device from the drop-down menu....oh wait, it's completely blank.

My 2nd monitor works totally fine otherwise--my desktop is spread across both, so I'm not sure what's going on here. Any ideas?