Sometimes you have to load a logical volume that is not part of the main filesystem. Other times you might have to mount a volume from a live CD, perhaps to recover data from a system that is no longer booting.
In both cases it needs to be done manually.
Install the LVM tools
If lvm2 is not already installed:
sudo apt-get install lvm2
Scan for and activate volumes
If you can see the disk with fdisk but LVM does not recognise the volumes, the kernel may not have scanned for them yet. Start by scanning for physical volumes and volume groups:
sudo pvscan
sudo vgscan
sudo lvscan
pvscan detects LVM physical volumes, vgscan finds the volume groups, and lvscan lists the logical volumes within them.
Activate and mount
If the volume group is inactive (common after booting from a live CD or moving a disk to another machine), activate it:
sudo vgchange -ay
This activates all available volume groups. Now mount the logical volume:
sudo mount /dev/VolumeGroupName/LogicalVolumeName /mnt
Replace VolumeGroupName and LogicalVolumeName with the names shown by lvscan. If you are not sure what the volume is called, check /dev/mapper/ for device-mapper entries.
Troubleshooting
If lvscan shows your volumes but vgchange reports “Volume group not found”, check the output of vgscan carefully. Sometimes the volume group name contains a hyphen (e.g. my-server) which can confuse the command — try using the full path from /dev/mapper/ instead:
sudo mount /dev/mapper/my--server-myvol /mnt
Note the double hyphen — LVM represents the hyphen in the volume group name as — in the mapper path.
Enjoy!