• There exist some solution to move a time machine data folder to a new drive by making a clone of the drive. My problem is that I already have data on the new drive and that this data can difficultly be moved.

• It try (after switching off Time machine of course) with rsync while keeping hard links:

sudo rsync -av -H  /Volumes/tera_enigma/Backups.backupdb /Volumes/2T_un/

• This takes a while as rsync has to manage a lot of them (may take also a lot of memory, so be patient)