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I have a Virtualbox VM, with Ubuntu 12 installed in a 8gb disk, with encrypted LVM.

I managed to add a 20gb disk, and now I need to expand the LVM to use those 20gb too, or just copy the entire small disk to the big one and then remove the small one.

How do I do that?

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Assuming the new disk is /dev/sdb, the following is the approach. Make sure you fully understand each of these commands before doing anything:

Create a new physical volume with the disk:

pvcreate /dev/sdb

Add the volume to your volume group:

vgextend <vg> /dev/sdb

Move anything from the 8GB disk to the 20GB (assuming it is /dev/sda)

pvmove /dev/sda /dev/sdb

Remove the 8GB from the volume group:

vgreduce <vg> /dev/sda

Stop /dev/sda being a lvm physical volume:

pvremove /dev/sda

Then you can take it out of the box...

Use the pvdisplay -m commands to confirm what is on each disk:

pvdisplay -m /dev/sda
pvdisplay -m /dev/sdb

You want to be certain nothing is left on the disk you remove.

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Should this work fine even if the partition was encrypted? I didn't have /dev/sda, I had /dev/dm-0. I used that and everything seemed to work. But then I rebooted and got locked out, couldn't go past through the 'enter passphrase:' prompt. That was without removing the old disk in virtualbox. When I removed the old disk, I just got something like 'no bootable disk found!' (also tried moving the new one to port 0). (don't worry, it was a test vm) –  HappyDeveloper Jul 5 '12 at 5:24
Ah - you mean you have LVM on top of an encrypted disk, rather than the LVM volumes encrypted? –  Paul Jul 5 '12 at 6:25
When I was installing Ubuntu 12 I chose 'encryped LVM' –  HappyDeveloper Jul 5 '12 at 7:13
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