I'm not sure how helpful this method is. When I first got my Oppo I did started out using a flash drive plugged into the front USB. Then I moved up to a 3TB EXT HDD. I know, I know, it's crude & primitive compared to others set ups around here but it works for me.
Anyways as usual it has a power/activity light on it. I noticed that even with the Oppo turned off, the HDD light still showed as on. And I can pick up the HDD & feel the disc spinning. So simple powering down would not benefit a flash drive or HDD, just unplugging it. And of course the question returns how to do that safely.
For my self music & movies are usually in the evening & I just pull the USB plug from the Oppo when I'm done. So far, so good.