Why won't my USB drive power down on eject?
Use this Terminal command  sudo lsof | grep DODO

to find out who is using the USB.

Wait for it to respond, it might take 10 seconds.

If no one is using it, it is safe to eject the disk (pull out).

I did not answer your question "Why it does not Power Down" ? I do not know, probably missing the handshake saying I am done.