Question was asked again recently. And in that specific case the recording was disrupted before anything was written to disk. If the amount of data in the "would be bag" is smaller than the "chunksize" attribute of the bag then all the data would have been lost and there would be nothing to recover.

--chunksize=SIZE

Advanced. Record to chunks of SIZE KB (Default: 768). This is a buffer within the bag file object. Lowering this value will result in more writes to disk.

This question is quite old and the answer might not apply to this specific case, but it has quite a bit of views so I thought I would share my answer here.