"Disk Not Ejected Properly" error after El Capitan

It's back ... Had the same problem for a few months after installing Yosemite ... but it eventually went away.

Now getting this error after installing El Capitan last week. Problem surfaces after my iMac sleeps.

Using a Lacie 2TB external on USB3 for Time Machine.

If anyone has ideas, similar problem or a fix, please help ... thks.