Do you store your Enterprise Vault data on locally attached disks to your server (DAS)? Do you use a Storage Area Network (SAN)? Do you use Network Attached Storage (SAN)?
Whatever you use, the storage has to be fast, and reliable.
On top of that the storage has to be flexible and cope with the increasing footprint of Enterprise Vault over time. Not just more email being archived, but think of all the different content sources and how they might be added on to Enterprise Vault at some point in the life time. Things like SharePoint, File Server Archive, Domino, Public Folders, and more.
Think also of how long that 'life time' might be. Many people launch in to using Enterprise Vault without a clear strategy when it comes to storage expiry. Of course that's really a mistake, but it is a very common one! Without any kind of storage expiry not only does the footprint of Enterprise Vault grow...