@LittleTux There are a few ways to go about this. One method is like the tutorial you found about adding a second (logical) storage node to the FOG server. This method has its good and bad sides. On the good side its pretty easy to implement as long as you follow all of the steps. On the bad side now you have to decide when you create the image definition which disk you want to store the image on.
One alternative would be to mount /dev/sdb1 on /images. That would make your /images directory 1,2TiB in size. (there are a few steps that need to happen to make this configuration, but no changes are needed in FOG).
The second alternative is to mount /dev/sdb1 below the /images directory. Like
/images/administrateur When you create the image definition you again have to decide to put the images in /images or /images/administrateur directory one is on /dev/sda and the other is on /dev/sdb.
In my opinion alternative #1 is the best option because it has the least impact on FOG and its settings. If you need to expand the VM disk beyond 1,2TiB you can do that pretty easy as long as /dev/sdb1 is a normal partition.