The GridIn.write change in 2.0 introduced a serious regression that causes loss of data when you write strings and at any time you have _buffer filled up completely, any data after that point will be ignored. Since my code was chunking write calls with chunk_size (I needed to do additional data processing before writing to GridFS) it meant that any file >256KB became truncated.

Please fix this ASAP with the patch attached, which fixes the problem and improves chunking with writes.