Interesting, I just ran into a similar issue with Lightroom 4.3. I've been using LR since early in its development and this is a first.
An import of ~30GB worth of RAW (NEF) images stalled about the 25% mark with a 'disk full, create 250MB of space' type error. I checked and there's over 200GB of space remaining on the target 1TB drive. I simply hit the 'retry' button and away it went. The problem hasn't reappeared yet.
Perhaps it's the 20% threshhold issue mentioned above? (hadn't heard that one before) or perhaps it's a r4.3 bug?
regards,
Phil