Logical Volume Management is used to treat seperate disks as one. However, this can cause trouble when trying to access the disk from a recovery CD, etc.
To mount lvm partitions from a livecd (for purposes of recovery/emergency maintenance), simply:
modprobe dm-mod to load required modules,
lvm vgchange -ay to become aware of the volume groups, and then mount as normal with
mount /dev/vg_(machine-name)/(distro-name) /location/to/mount/to.