Thanks guys great. Yes when look at dock, is seems nothing using is. Ok will try to remember to look at activity monitor, but monitor wont way what using app either right?
Activity Monitor tells you what tasks are reading and writing to a disk. and it tells what user the task belongs to. It does not tell you what disk or what volume the task is reading or writing to. In the case where a task quits without releasing the drive then Activity Monitor may not tell you anything. There used to be a Terminal command that would tell you what tasks are accessing what volume, but either it has been deprecated and removed from macOS or I forgot what the command is/was.
There are any number of both system and user tasks that may be accessing any given drive at any given time. The most common system owned suspects are Disk Utility, Time Machine and Spotlight. Among user tasks, Carbon Copy Cloner, Hazel, SpamSeive, Keyboard Maestro, and almost any app that monitors the system in any way have all been implicated at one time or another.
If this is happening often Spotlight would be high on my list of suspects and the classical "fix" for anything to do with Spotlight is rebuilding the Spotlight index following
Apple's Instructions then leave your computer running, go to bed, get a good night's sleep and by the time you wake up the next morning it should be finished.
