After Windows 10 performed an update, I couldn't map my DNS-323 NAS drive. Bottom line, here is what I found. The Windows 10 update apparently reset one of the security settings required. The following security check box MUST be CHECKED.
Go to: Control Panel / Programs and Features / Turn Windows features on or off
Insure the box is CHECKED for SMB 1.0/CIFS File Sharing Support. This older drive needs this box CHECKED.
I hope this helps out with your older legacy product NAS product.