Linux – Copy/clone data from bigger HDD to smaller one

I'm unclear if you're now missing a single 1.8TB LV or a 1.8TB PV + VG + LV. If the LV was located in a different VG, then best try to located the missing disk, starting from pvscan. You might for example just have a lvm filter or cache issue. Many distros try to make you hurt a lot by adding lvm.conf to the initrd, but not telling you you'll need to rebuild it if you change it later on. You can run pvscan -vvv and see if it says anything about "ignored by filtering".

If you just lost the LV from the config, then the question is how that happened and if it's still readable. So a test dd from the disk into /dev/null might be a good starting point, i.e. see if you can read "roundabout where the old LV was located"

Normally though I'd first make the system boot successfully again, by commenting out the affected LV in fstab.

As a last-est resort you can find the last LVM config backups on your disk. One can read it using something like dd + strings + grep -A1000 "LVM". But you're not that lost yet :) Based on that config one can re-create the older states of the LVM config to disk. But before doing that you have to be clear about whats corrupted and I would test the whole thing on a copy of the affected disk, not on the original one.

It seems to work, without any problems.

What I did:

1. add the new drive to the RAID and let mdadm sync the drives
2. remove the new drive from the RAID again after sync is done
3. run fsck. on the drive
4. mount the drive and test some directories and files manually
5. unmount and re-add the drive to the RAID

All these steps worked without any problems. The filesystem worked as expected.

So the answer finally is (without guaranteeing its generality): You can add a device with differing block- and I/O size to a RAID1.