exactly the same issue here… it is driving me nuts.
i have a new 4TB WD blue and issues with it going into sleep mode (spindown).
it is configured as drive E: and a pure DATA disk.
i run windows 10 64 bit.
i also tried hddscan and manuall spindown works.
i replaced a 3TB WD green that had no issues going into sleep mode … it worked just fine.
i know how to setup energy saving features in windows 10 and i made sure that the system is not accessing the 4TB WD blue, preventing it from going into sleep mode.
there is no cache or swapfile on E: there is no tool accessing the drive.
again… when i replace the 4TB BLUE with the 3TB GREEN all works as expected.
the drive is a DATA drive as i wrote so it could be idle most of the time.
and i hear the annoying noise the drives motor makes, as i have an absolute silent system otherwise.
the GREEN is a WDC WD30EZRZ-00WN9B0
the BLUE is a WDC WD40EZRZ-00WN9B0
im pretty much clueless why this drive won´t go to sleep (spindown).