I have a strange problem when making a backup of my mysql databases. In order to do so I have a 2TB external hard disk mounted when needed under /backup.
Before making the backup I stop mysql (with /etc/rc.d/mysqld stop) and use rsync to copy all the data to the external hard disk.
As soon as the copy is done I start mysql again (/etc/rc.d/mysld start). The whole copy process takes about 20 minutes (all databases are ~60GB).

But... when I try to umount the backup unit I get

root@slackserver:/# umount /backup0
umount: /backup0: device is busy
umount: /backup0: device is busy

And when looking who is keeping the device busy (backup0 = /dev/sdf1):

root@slackserver:/# fuser -m /dev/sdf1
/dev/sdf1:           22420c
root@slackserver:/# ps auxw | grep 22420
root      5630  0.0  0.0   2056   644 pts/1    R+   17:03   0:00 grep 22420
root     22420  0.0  0.0   2760  1376 pts/0    S    11:13   0:00 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql --sql-mode=ansi --pid-file=/var/run/mysql/mysql.pid

The only way to umount the backup unit is to so before restarting mysqld.

But why is mysql keeping the device busy. All other hard disks are not "locked" by mysql. Does mysql see the data on the backup drive and recognize it as mysql databases?
As you can see, the datadir is set to /var/lib//mysql and that is (on my system): /dev/mapper/vg2-sql on /var/lib/mysql type xfs (rw) That is part of a software raid 5 with lvm 2 (device /sd[bcde]1).

What is going on here?

Have posted this question on expert-exchange as well and have received an answer.

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