Windows (especially Vista) is generally rather fickle about this type of thing. I’ve found it’s not even above saying that a file is in use when no applications are open. Anyways…
Do you have that file selected in After Effects when you’re trying to overwrite it? It could be that generating the thumbnail is causing the problem.
Other options include, shutting down After Effects, doing the overwrite and starting AE back up. Or importing the media with a new name and do an Alt-drag replace in the timeline. This last one stops being a viable option if you have multiple instances of a the clip to keep track of.
Basically what I’m saying is that this is probably more of a Windows thing than an AE thing.
All the best,
Ben
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