The Sedona Conference® Working Group 1 recently published a Commentary on the issues of preserving, managing, and identifying not reasonably accessible electronically stored information or “NRA ESI". The result is a five-step framework for analysis and six Guidelines for making reasonable, good-faith assessments where no “bright line” rules exist. Ira P. Rothken is a co-editor of the above Sedona Conference Publication entitled Commentary on Preservation, Management and Identification of Sources of Information that are Not Reasonably Accessible.which can be downloaded here and here is a web seminar  on the topic. You can find a review and summary of the commentary here in which the author states that "...I highly recommend that you read this new publication and, more importantly, consult it when faced with tricky preservation issues."