USE THIS INFORMATION AT YOUR OWN RISK!!
Assuming you are sure you know which disk you added, this is how you expand...
1) Make the disk LVM aware
pvcreate /dev/sd<x> [if you have a problem you can force with -f]
2) Add the Disk into the Volume Group
vgextend <Volume Group Name> /dev/sd<x>
3) Extend the Logical Volume
lvextend -L +yG /dev/<Volume Group>/<Logical Volume> [ Where y is the size in GB of the new disk ]
4) Extend the filesystem
ext2online <mountpoint>
Assuming you are sure you know which disk you added, this is how you expand...
1) Make the disk LVM aware
pvcreate /dev/sd<x> [if you have a problem you can force with -f]
2) Add the Disk into the Volume Group
vgextend <Volume Group Name> /dev/sd<x>
3) Extend the Logical Volume
lvextend -L +yG /dev/<Volume Group>/<Logical Volume> [ Where y is the size in GB of the new disk ]
4) Extend the filesystem
ext2online <mountpoint>