1st off, don't use fdisk to partition the drive. fdisk cannot deal with disks larger than 2TB. (Does not support GPT)
You should use parted instead, or rather the GUI version gparted.
You can get gparted by adding the EPEL repository
If you don't want to install gparted, here's howto on parted
Using GPT disks will allow RHEL5 to use disks larger than 2TB. But take note that you will not be able to boot to this disk.