Can't umount on RHEL/CENTOS Error: device is busy

[root@sandbox1 ~]# umount /dev/sdb1
umount: /mnt/pen: device is busy
umount: /mnt/pen: device is busy

Solution :

[root@sandbox1 ~]# lsof | grep /mnt/pen
esd       23711   root  cwd       DIR       8,17      4096    1228794 /mnt/pen/720p BRRip x264 - HDMiCRO by Mr. KickASS
esd       23711   root    3r      REG       8,17 942758269    1229005 /mnt/pen/ - HDMiCRO by Mr. KickASS/ Mr. KickASS.mp4

[root@sandbox1 ~]# kill -9 23711


[root@sandbox1 ~]# umount /dev/sdb1

Done.