How to check which process is using the external harddisk / pendrive / thumb drive

sudo fuser -m /mnt 

(it will list out the processid)

sudo kill -9 processid

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https://nomacs.org/ 

How to mount and umount the device in ubuntu?

to mount the device:

sudo lsblk

sudo mkdir /mnt

sudo mount /dev/sdc1 /mnt

to umount the device

sudo umount /mnt

or 

sudo umount /dev/sdc1

sudo udisksctl power-off -b /dev/sdc1