The DiskStore as it stands now is not very useful. It requires an in-memory index of items in the store which means it can only be used for one server at a time and is useless across server restarts. This seems to totally defeat the purpose of an external store if it can't be shared across several servers and persist its data over a restart.
Rework the DiskStore to rely only on the file system for its indexing. This means it may not be possible to track the metadata as easily, or perhaps a second file will need to be written to contain metadata.