I''ll just poweroff NAS once at month and plug second HDD and make backup
... It looks this was is working ...I bought second hdd (now primary WD RED) only because I needed to put HDD from my PC to NAS and not lost all the data ... (NTFS > EXT4)
And I bought this NAS because I made custom waterlooling and HDD (WD Blue 5400rpm) was the loudest thing in PC.