After backing up the catalog, Lightroom helpfully informed me of low space on the C: drive. This is what Explorer thought of the situation:
Or, in command prompt-speak:
Lightroom quirk or Windows quirk?
February 5, 2010
January 12, 2008
RAW Windows
One of the problems with RAW formats has been the need to use specialized tools to work with the files. Pro photo tools now typically understand many RAW formats, but that doesn’t extend to all the other apps that I want to use with photos, or use photos in. In Windows Vista (or more precisely, WIC) that’s been made a lot easier by support for image codecs for RAW formats. You can grab a codec for the RAW format used by your camera and then any app that uses WIC to access images gets automatic support for that file type. Great for users and also great for developers, who no longer have to worry about loads of complex code changes just to support the latest RAW format variant.
There’s a list of codecs available from different manufactures on the Microsoft Professional Photography site [link] which, interestingly for me, also includes a link to a DNG codec.
Although I shoot with Canon (and therefore .CR2 RAW files), for various reasons I convert to DNG. At the moment that results in a hybrid workflow where I shoot RAW+JPEG because some apps I use don’t like DNGs. Looks like we might be getting closer to the time where I don’t need to do stunts like that 
so I’m off to play with codecs for a bit!