Grab Still = Black Frame

Resolve is doing it again. I switched to Mountain Lion and that seemed to fix the problems I was having with Resolve previously, which include the stills I grab coming up only as black frames. That problem, however, has returned. Does anybody have any ideas why this would happen and how to fix it? Frankly, this is getting pretty ridiculous when I can't even use a still to do a reference wipe and match shots and instead have to jump back and forth between shots in the timeline.

[Peter Chamberlain]

Stills are stored on the drive you list first in preferences. This needs to be a fixed location or you won't be able to consistently save and recall stills.
What drive do you list there? Do you have read/write permissions for that drive?
Peter

Thanks for the reply Peter. I looked into what you suggested and it did turn out to be an issue related to that, but I still can't figure out why it happened to begin with. In my Preferences, an external drive that is not always connected had taken the first spot without me intending for it to do so. I was accustomed to stills being saved to the desktop. My desktop was listed last, and it seemed that the only way for me to move it to the top was to delete all the others, which I did. Once I did that, the stills function in Resolve worked again. However, what is strange is how the drives became reordered without me intending to do so and why, even though it was an external drive listed first, Resolve was still saving the still files to my preferred location (desktop), except that those files were completely black. I guess it's a mystery that doesn't truly need solving, because the stills function works again and that's the important thing, but it does leave me a bit curious.

Thanks for your help!