Migrating data to a SAN is a major undertaking, but it is also a good time to do some housekeeping and review data-retention policies. This is especially true if you are categorizing data by how long it has been since it was modified.
Data that hasn't been modified in a year or more is usually considered the least important to migrate to a SAN. It is also a prime candidate for being moved to less expensive forms of storage, such as tape.
More generally, the time stamp reports generated in analyzing the data for movement to the SAN provide a quick review of whether data is being retained in an efficient, economical manner. Like a lot of tuning, data retention is often an iterative process where policies are made and then modified in light of results. For example, the date-stamp analysis may show that storage savings could be achieved by moving some classes of data to tape after 30 days rather than waiting 180 days, because the access levels are almost the same after 30 and 180 days.
Sun Microsystems has a white paper titled "Storage Resource Management: A Practitioners's approach" available on its web site which discusses this and other issues.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.