How to check what are using the mounted disk when I want to eject it

The command you are looking for is fuser.

It displays all processes accessing /media/SDD by typing sudo fuser -mv /media/SDD, where the m tells it to look on the given location, the v switches the output to a human readable list instead of just a bunch of PIDs.

To automatically kill all processes (!! Use with care !!) accessing the directory, run sudo fuser -mvki /media/SDD. k means "kill" and i gives you a 2nd chance and lets you confirm each kill (which means you also can leave it and just type ...-mvk... to kill them without asking again).

To do further research on the fuser command, please read its manual by typing man fuser or online at manpages.ubuntu.com.

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