Thanks a lot for your input guys. One thing I had omitted to tell is that the cloned version is on a dedicated 500 Gb partition, not the whole volume. I did try several time to erase, repartition, reclone; no luck.
LaCie seems to clearly indicates that the drive is “bootable in Firewire for Mac”, as shown here and there.
I tried all FW800 connections, before startup or at (option-key) startup.
As I said earlier, I won’t return it as I needed storage anyway. I’ll try the commands you provided later when I get a hand on the drive again.