From the MS Exchange team blog:

NOTE: Clustering does not immunize Exchange against failures on a shared disk. Shared disks are virtual server resources, and will fail over with the rest of the Resource Group. Although Exchange services will continue to run, clients will not be able to access existing email until a shared disk problem has been corrected.
Clustering provides redundancy for Exchange as an application, but not for Exchange data. Therefore, regardless of whether you are clustering Exchange or running Exchange on a standalone server, it is critical to design disk systems for fault tolerance and redundancy.
And to think Microsoft marketing still attempts to paint the Lotus Domino shared-nothing clustering model as a waste of disk space.  Of course, they also often forget to point out that Domino on Windows can take advantage of the same Windows clustering services as Exchange does, meaning you can go with their approach or go with the superior Domino clustering approach.
Update: I forgot that there are a bunch of readers here who might not be familiar with Domino clustering.  Domino can replicate data across clustered servers, meaning that multiple copies of the data can exist in physically disconnected ("shared nothing") environments.  Further, Domino has the unique advantage of allowing clustering on a per-mailbox or per-application basis.  So organizations can design different service levels based on user profiles or whatever criteria -- not every mailbox on a server has to be clustered.

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