I hv just plug one new HD to the RH server , but when I run "df" , it is no difference (can't see the new HD is present ), could suggest how can I mount the new HD to the system ? thx

df only works on mounted harddisks

run fdisk -l /dev/hda
fdisk -l /dev/hdb
fdisk -l /dev/hdd

The primary harddisk is hda, and the secondary is hdb or hdd (depends on the way you connected it).

Anyway, first identify where it is connected, and then create partitions on it (if not created already). use fdisk to create partitions. Then mount the partitions created with mount /dev/hdd1 /mnt/hdd1

Be sure to create a mountpoint (like /mnt/hdd1 or such) before mounting, or the mount command will complain about the mountpoint not being found.