e2fsck shows filesystem in use but it is not mounted

e2fsck commnads: 

sudo e2fsck -fvy /dev/sdb
 

shows 

e2fsck 1.46.5 (30-Dec-2021)
/dev/sdb is in use.
e2fsck: Cannot continue, aborting

But /dev/sdb or /dev/sdb1 is not mounted. There must be some program using it.

Use the command to check:

sudo lsof /dev/ | grep sdb

It shows:

 photorec  24095             root    4r   BLK    8,16      0t0  426 /dev/sdb

which means PID 24095 is using the umounted /dev/sdb. Just kill it.

sudo kill -9 24095

Done.