With the release of Exchange 2010 SP1, administrators can now use separate Mailbox Databases to store the Personal Archives of users – this is particularly useful if you have some larger, slower (and probably by virtue, older) storage that’s not really up to the I/O of your Exchange Server (that old SAN/NAS sitting in the corner of the server room?). It’s also useful if you just don’t have the capacity on your main storage.