Windows 7 has issues authenticating to some NAS devices. I have seen this mostly as being asked to provide credentials, where none should be needed.
The root cause is NTLMv1 is disabled by default, in favour of NTLMv2.
To turn on NTLMv1 and NTLMv2:
Open MMC, add the Group Policy Object Editor snap-in for the Local Computer. Open Computer Configuration, Windows Settings, Security Settings, Local Policies, Security Options.
Find: Network security: LAN Manager authentication level
Change the setting to: Send LM & NTLM – use NTLMv2 session…
Reboot your computer and you should have access to your NAS devices.
Source: H2S04 at sevenforums.com