I always thought LVM (Linux's Logical Volume Manager) was kind of neat for the flexibility it gives you in adding and removing disks and resizing volumes such. However, in practice, I find it's usually more trouble than it's worth. It adds a layer of complexity between me and my data.
Often I need to mount a disk configured with LVM on another Linux machine or in an Ubuntu live CD environment. Out of the box the logical volumes aren't recognized, so I can't mount them.
It's fairly easy to add LVM support and mount the volumes though.
You install the lvm2 package, load the device mapper kernel module, and then activate any lvm volume groups on your disk.
$ sudo apt-get install lvm2
$ sudo modprobe dm-mod
$ sudo vgchange -a y
(assuming your disk with logical volumes is already connected)
$ sudo mount /dev/mapper/<logical volume name> /mnt
And that's all there is to it.
If you want to deactivate the volume groups (recommended before unplugging a USB disk with logical volumes):
$ sudo vgchange -a n
Warning: the above command will deactivate all volume groups, so check the vgchange manpage first, if that's not what you want.