Altering the Windows Registry
If a firmware upgrade is not available, short of spending money on a new NAS drive (or enclosure) you have a last chance to fix the problem using a registry hack.
To access the registry, open Start and type regedit, then press Enter. Agree to the User Access Control notice and the Registry Editor should open. Before making any changes, go to File > Export… and make a backup of your system registry, saving the file to a non-system disk in the event of a problem.
Next, browse to HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Lsa\LMCompatibilityLevel, and right-click the item, choose Modify… and set its value to 1; if you cannot find LMCompatibilityLevel, open the Lsa folder, right-click in an empty area and select New > DWORD (32-bit) Value, then set the value to 1.
You should also try 2 as an alternative value should this not work, but remember to restart both your Windows 7 computer and the NAS drive before retrying.