They are 4T portable G Technology. I didn't tried to turn off encription. I'm giving up on this. I'll reformat them and make a conventional backup, instead. Anyway, thanks for your help.
Yes, they are USB 3.0 capable drives. If I boot the machine with both drives plugged in, the result is the same: one of the drives will not show its volumes. Which, it seems random. I've changed the order in which I plug them, the usb port... Some times one of the drives will show the volumes...
I cloned a USB disk with encrypted partitions into another, identical one (same size, same model...). I changed the name of one of the partitions in finder for one of the disks, to be able to tell them apart. When I plug either of the drives by themselves, partitions are recognised fine. When I...