A couple of steps to take to add an additional drive to cPanel / WHM:

Find the path to the disk you are trying to add by issuing the following command:

fdisk -l | grep '^Disk'

Create a partition and format the partition to EXT3

fdisk /dev/sdb

and then use following in the prompt – "n" for new partion – "p" for primary partition – "1" for the first partition – "Enter" / "Enter" for the first AND last cylinders (automatically use the entire disk) – "w" to save what I have done

mkfs -t ext3 /dev/sdb1

cPanel picks up any directory similar to “/home” “/home2” etc, so the next step is to create another directory as follows:

mkdir /home2

The next step is to label the disk using e2label

e2label /dev/sdb1 /home2

You can now mount the drive using:

mount /home2

To enable the automatic mounting of the drive edit /etc/fstab and add the following:

LABEL=/home2 /home2 ext3 defaults,usrquota,noatime 0 0

Ensure that there is an empty line after this, as this is required by fstab

Information was referenced from the following sites:

How to format and mount second hard drive on Linux

How to mount additional Harddrive to CentOS Cpanel

Managing Multiple Hard Drives with cPanel