HDD being used 100% even when data files are not on that disk

I am facing very high disk IO . i have 2 HDD 1 is a sata disk and the other is a SSD .

The sata disk is showing 100% disk utilization .

My.cnf is as follows

datadir = /mnt/disk1/mysql socket = /var/lib/mysql/mysql.sock user=mysql

symbolic-links=0 innodb_file_per_table = 1 thread_concurrency = 8 query_cache_size = 32M thread_cache_size = 8 myisam_sort_buffer_size = 64M read_rnd_buffer_size = 8M read_buffer_size = 2M sort_buffer_size = 2M table_cache = 512 max_allowed_packet = 1M key_buffer = 1024M max_connections = 450 innodb_buffer_pool_size=4096M innodb_flush_log_at_trx_commit=0 log-slow-queries = 0 slow-query-log-file = /mnt/disk1/mysql/mysql-slow.log

[mysqld_safe] log-error=/var/log/mysqld.log pid-file=/var/run/mysqld/mysqld.pid

The /mnt/disk1 is a separate SSD disk but my other files on a sata disk .

The sata disk is showing 100% utilization. and iotop reads as follows

--basedir=/usr --datadir=/mnt/disk1/mysql --us~n/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 18011 be/4 mysql 3.77 K/s 7.54 K/s 0.00 % 0.02 % mysqld --basedir=/usr --datadir=/mnt/disk1/mysql --us~n/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 26755 be/4 mysql 0.00 B/s 15.09 K/s 0.00 % 0.00 % mysqld --basedir=/usr --datadir=/mnt/disk1/mysql --us~n/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 14472 be/4 apache 0.00 B/s 3.77 K/s 0.00 % 0.00 % httpd 14473 be/4 apache 0.00 B/s 3.77 K/s 0.00 % 0.00 % httpd 26815 be/4 mysql 0.00 B/s 18.86 K/s 0.00 % 0.00 % mysqld --basedir=/usr --datadir=/mnt/disk1/mysql --us~n/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock 26662 be/4 mysql 0.00 B/s 18.86 K/s 0.00 % 0.00 % mysqld --basedir

I am confused what is cause of this high disk usage .

Thanks