The default local security policy on Windows Vista is set to use NTLM v2 only. After some off and on troubleshooting I finally discovered this was preventing me from accessing my Western Digital NetCenter NAS. The following procedure changes the policy to allow older Lan Manager Protocols if needed:
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Run MMC snap-in secpol.msc
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Expand Local Policies -> Security Options
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Find Network security: LAN Manager authentication level
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Double click and change to “Send LM and NTLM – use NTLMv2 session security if negotiated”