While trying to unmount a secondary drive I see below error

umount: /backup: device is busy.

(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))

The issue was due to the drive /dev/sdb being used by some process

Below is the process on how to find process using drive and kill it and umount drive

# Command to unmount drive

root@oM1050 [/backup/weekly]# umount /backup
umount: /backup: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))

#command to see disk listing and name on which its mounted

root@oM1050 [/backup/weekly]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda3 48G 20G 26G 44% /
tmpfs 16G 0 16G 0% /dev/shm
/dev/sda1 865G 537G 285G 66% /home
/dev/sdc1 118G 93G 19G 84% /var
/dev/sdb 688G 686G 0 100% /backup

#command to find the process number which is using the drive

root@oM1050 [/backup/weekly]# fuser -m /dev/sdb
/dev/sdb: 11676c 12801c

# checking which process is running on it

root@oM1050 [/backup/weekly]# ps auxw|grep 11676
root 11676 0.0 0.0 109644 3088 pts/1 Ss 07:23 0:00 -bash
root 19587 0.0 0.0 105372 892 pts/1 S+ 07:32 0:00 grep 11676

# checking which process is running on it
root@oM1050 [/backup/weekly]# ps auxw|grep 12801
root 12801 1.2 0.0 106756 4544 pts/1 D 07:24 0:06 rm -i -rf 2018-02-14
root 19843 0.0 0.0 105372 896 pts/1 S+ 07:33 0:00 grep 12801

# killing the process which is engaged in /dev/sdb

root@oM1050 [/backup/weekly]# kill -9 12801
root@ok1050 [/backup/weekly]#
[1]+ Killed rm -i -rf 2018-02-14

and finally drive unmounted

root@ok1050 [~]# umount /backup
root@ok1050 [~]#