Storage management is simultaneously maddening, rewarding, and baffling. The past few years in particular have brought a wealth of disk and storage technologies, including faster disk drives, Fibre Channel Storage Area Networks (SANs), and Network Attached Storage (NAS) devices that make adding storage to a network as simple as plugging in a box. And over the past 20 years, the cost of raw storage (i.e., bits on a disk) has dropped from $1000 per megabyte to less than $1 per gigabyte--that's roughly a millionfold improvement!
In the file and print service world, inexpensive NAS devices have become increasingly popular. These devices typically combine low acquisition cost (compared to servers, anyway) with easy expansion and minimal maintenance--two characteristics that Exchange Server administrators are always hunting for.
There's a fly in the ointment, though: Microsoft doesn't support most NAS devices for use with Exchange. The network latency that the devices inevitably introduce can cause problems, especially when you keep transaction logs and databases on separate volumes. In addition, some NAS vendors introduce their own redirectors (or other components) between the file I/O APIs that Exchange uses and the Windows I/O Manager in the kernel. Although this kind of driver stacking is useful in many circumstances, it adds supportability complications (not to mention that an unstable driver can cause a lot of other problems). The Microsoft article "XADM: Issues That Might Occur If You Place Exchange Data Files on Network Shares," http://support.microsoft.com/?kbid=314916 , explains the potential risks in greater detail.
NAS devices generally operate in one of two modes. A block-mode device appears as a big disk drive, much like a SAN would. The fact that a network--instead of a SCSI, IDE, or Fibre Channel interface--carries individual blocks of data to the NAS device is supposed to be invisible to applications. A file-mode device, which most people identify as a "filer," looks like a big file share to the network and to users.
How can you tell whether Exchange will support a particular device? The only authoritative way is to check the Windows Hardware Compatibility List (HCL) at http://www.microsoft.com/hwdq/hcl/search.asp . The HCL lists devices in their supported configurations. Why the emphasis on configurations? Because some vendors sell devices that can operate in multiple modes. Microsoft supports some block-mode devices for use with Exchange but doesn't support any file-mode devices, no matter what various vendors might claim. (In fact, Exchange includes checks that prevent you from moving databases onto a mapped network drive or a UNC path.)
If you're interested in using a certain device, read the HCL to see whether Microsoft supports that device for use with Exchange. You can ask NAS vendors for information too, but to avoid expensive mistakes, be sure to cross-check a vendor's story against the HCL. The Microsoft article "XADM: Exchange 2000 Server and Network-Attached Storage," http://support.microsoft.com/?kbid=317173 , describes Microsoft's support policy for NAS devices; be sure to read this policy before you make a purchasing decision.