A method and system for storage of unstructured data in external data storage uses low-cost, minimally-functional external data stores (EDS) to store immutable, unstructured content. An external storage layer (ESL) interposed between an e-discovery management application (EMA), and the EDS constitutes an intermediary allowing access to external storage from the EMA and adding functionality unavailable on EDSs, offsetting the functional sacrifice incurred by using the EDS and preserving cost advantage. Caching content on the ESL during propagation to the EDS eliminates latency during file propagation. The ESL creates metadata and maintains an index of the data, allowing the data owner to search and retrieve from the EDS. The ESL compresses, decompresses, encrypts and decrypts data. An ESL vendor can service a number of clients on a fee or subscription basis. The ESL can distribute client data across EDSs and mirror data stored on a first ESL on another ESL.