Sometimes you have to load a LV (logic volume) that is "not part of the main file system".
Other times you might have to mount a LV from a live CD, maybe to recover some data or a system that is not booting anymore (which is my case right now).
In both cases it needs to be done manually.
Install the LVM package in case you haven't done it yet:
sudo apt-get install lvm2
Then, if you can see the disk (eg: with fdisk) but lvm doesn't allow you to mount it check if the LV is active.
To do this you have to list all LVs and check if the "a" attribute is listed under the column "Attr" for the partition you are interested in:
In case the LV you want to mount is not active you can activate it with:
sudo lvchange -a y /dev/volume_group/logic_volume
Now it's ready to be mounted normally:
sudo mount /dev/volume_group/logic_volume /mnt/my_path