I've been trying to get this to work all day with no luck. I need to reinstall/upgrade the OS but would like to keep /data and /home partitions intact.

Using Ubuntu Server and the disks are LVM.

I can accomplish my goal if I run a manual install/reinstall of Ubuntu and run the manual disk partitioning but I have a few hundred servers to do, so it's not ideal. I can also afford to lose the /data, it's just a pain to replicate TB of data.

Has anyone got this to work? I've found very little help online. I find it hard to believe it's so easy to do manually but no easy way to do with with a preseed config.

I've found something that works for me. I've replaced all the partman options with this:

You have to manually add back into fstab but the data remains after a reinstall. I'll probably add that logic to the preseed file itself.