Not sure what SBS recommends, but Microsoft is another story.. ;\)

SBS is really specialized - can only be one SBS in a domain, it must be the PDCe, and while it does support additional DCs, well, to quote their documentation:
  • You can install a computer as a BDC in an SBS domain, but there is minimal advantage in doing so. Because the SBS server must function as a PDC, the BDC only provides redundancy for authentication, not fault tolerance as in a traditional Windows NT domain where a PDC does not act as an applications server.
  • Using a BDC for load balancing: In a domain where there are 25 or fewer users, one domain controller, the PDC, can easily handle domain validation.
  • Logon scripts should be replicated to the BDC. In an environment where clients could be validated by a server other than the PDC, such as a BDC, all logon scripts should be replicated to the BDC or Client Setup will fail.
  • Using a BDC in case the PDC goes down: In an SBS domain, if the PDC goes down, whether or not the BDC is promoted, the users will be able to get validated. However, users will not be able to access applications other than those installed on the BDC, because none of the applications from an SBS server can be installed on any other server except the SBS server.
  • If the BDC is promoted and the SBS server needs to be reinstalled, it cannot be installed into the same domain as a BDC and then promoted because SBS installs as a PDC only. If the PDC ever is reinstalled without a full restore from a backup, the user accounts and machine accounts will have to be re-created and the BDC will need to be reinstalled to become a member of the new domain.

I guess, if you run SBS, you need redundant disk and good backups, eh?

Glenn
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Actually I am a Rocket Scientist! \:D