Different speed between two goflex net harddrive

[tomcheng76]

On the OS disk, i get
$this->bbcode_second_pass_code('', '[root@alarm ~]# dd if=/dev/zero of=test.file bs=1024k count=512
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 203.253 s, 2.6 MB/s')
The another drive, i get
$this->bbcode_second_pass_code('', '[root@alarm data]# dd if=/dev/zero of=test.file bs=1024k count=512
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 11.0021 s, 48.8 MB/s')

i suppose there shouldn't be a huge speed difference even i am having the OS running ?
both are goflex 2.5" disk, same model.

The blkid shows
$this->bbcode_second_pass_code('', '[root@alarm data]# blkid
/dev/sda1: UUID="749a7cc3-000a-44c8-a2af-3746abbb0ff8" TYPE="ext3" LABEL="rootfs"
/dev/sdb1: LABEL="data" UUID="e4ed6f45-6bdf-498e-bae1-fea513a42c80" SEC_TYPE="ext2" TYPE="ext3"')

How do i check if PIO is running ?