I’m not sure when this issue stared. Maybe with latest os x update.
When I boot my mac mini, I hear a " click, click, click " noise coming from USB 3.0 WD My Book 1140 Media ( description of disk utility ).
It happens the same thing if i disconnect power from disk, boot os x, and reconnect external disk.
After disk boots I don’t have any other issue.
This is a Time Machine disk, no other files on it.
I used disk utility repair from recovery but didn’t solve the issue.