Activating second disk on ubuntu

I have two disks installed on my ubuntu dapper machine. Each is 250GB. But only one of them appears to be mounted and usable.

root@weasel:/var/log# df -kh
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/Ubuntu-root
                      224G   87G  125G  42% /
varrun               1007M  208K 1007M   1% /var/run
varlock              1007M  4.0K 1007M   1% /var/lock
udev                 1007M  108K 1007M   1% /dev
devshm               1007M     0 1007M   0% /dev/shm
lrm                  1007M   22M  986M   3% /lib/modules/2.6.15-26-amd64-k8/volatile
/dev/sda1             228M  102M  115M  48% /boot

Logical volume manager (LVM) has something to do with this.

root@weasel:/var/log# fdisk -l

Disk /dev/sda: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          31      248976   83  Linux
/dev/sda2              32       30401   243947025    5  Extended
/dev/sda5              32       30401   243946993+  8e  Linux LVM

Disk /dev/sdb: 250.0 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1       30401   244196001   8e  Linux LVM