Why does Windows say my NAS username or password is incorrect?

When you're sure the password is correct but Windows still rejects it, the problem is usually how Windows is handling the login, not the password itself.

Cause 1: guest access is blocked

Windows blocks unauthenticated guest logons by default. If the NAS share is set to allow guest access, Windows won't fall back to guest after your credentials — and the failure can surface as a credentials error. Fix: create a real user account with a password on the NAS and connect with it, rather than relying on guest.

Cause 2: stale cached credentials

Windows caches network credentials. An old, wrong entry for the NAS can override what you type. Clear it in Control Panel → Credential Manager → Windows Credentials, remove any entry for the NAS, then reconnect and re-enter the correct credentials.

Cause 3: connection method

• Connect using the NAS hostname, not its IP (an IP connection forces NTLM and can fail differently).
• Enter the username in the form the NAS expects (sometimes NASNAME\username).
• Confirm the account isn't disabled or locked out in the NAS control panel.

What not to do

Don't enable insecure guest logons on Windows to get around this — Microsoft warns it exposes you to spoofing and malware. Set up a proper NAS account instead. And back up the data behind the share regardless.