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Some NAS solutions support Active Directory, so you may want to check out QNAP and Buffalo to see which of their systems best fit your needs. Some folks don't worry about security on the LAN but so many times I hear stories of an untrained temp worker or disgruntled employee losing / deleting files that they shouldn't have had access to in the first place. Active Directory can keep your employees out of files they don't need to be messing with, and its granular approach to security makes the most sense for business-class networks, not home workgroups.