Exchange Server 5.5 could use a Network Attached Storage (NAS) device, provided that latency was sufficiently low. The official Microsoft position was that you could use devices that appeared on the Windows Hardware Quality Labs (WHQL) compatibility list. Exchange 2000 Server doesn't support NAS devices at all because the Exchange 2000 Installable File System (IFS) driver that makes the Exchange store visible as a file system can't function with a network redirector. The Microsoft article "Exchange Server and network-attached storage" (http://support.microsoft.com/?kbid=317173) sets out this limitation quite clearly, but several clever vendors built unsupported NAS devices that could be made to work with Exchange.
Exchange Server 2003 didn't support NAS until recently. The Microsoft article "Microsoft support policy on the use of network-attached storage devices with Exchange Server 2003," http://support.microsoft.com/?kbid=839687, describes the requirements for Exchange 2003 support. For more details, see the Web-exclusive article "Exchange and Windows Storage Server, Together at Last," April 2004, http://www.winnetmag.com, InstantDoc ID 42384.