Moving mysql database to new partition with different partition "type"
You could omit partitioning /dev/sdb and just put a filesystem on it.

mkfs.xfs -L MYSQLDATA  /dev/sdb


(other filesystems like ext3/4 will also work).

And in /etc/fstab

LABEL=MYSQLDATA /var/lib/mysql  xfs    defaults 1 2


To migrate:

systemctl stop mysql.server
mount /dev/sdb /mnt
chown mysql: /mnt
mv /var/lib/mysql/* /mnt/
umount /mnt


Then use:

mount /var/lib/mysql
restorecon -Rv /var/lib/mysql # if you have selinux
systemctl start mysql.service