My WD 2TB USB3 My Passport Ultra (powered only by USB) keeps blinking on macOS Sierra, including on today's 10.12.1 update. I've confirmed that this happens on my iMac and MacBook Pro, but does not happen under El Capitan. It would appear that the device is being written to, and is never allowed to go to sleep. The hard drive makes writing noises. I've made sure it is set as an exception in Spotlight, so it is not being indexed. Other WD drives aren't affected in this way and go to sleep normally. I've run First Aid, with no faults found and no changes to the behaviour. Anybody have any ideas? I've checked in Activity Monitor and can't see any processes that could be causing this. It's clearly a Sierra problem, but it's pretty much a deal breaker as the writing noise is too annoying to have to put up with all day.