I opened the HDD trap door (…) this drive t is simply not detected in BIOS.
The answers lays in the very thin (and consequently fragile) Sata HDD connector. The slightest handling manoeuver on it tends to tear it irremediably (other testimonials on the web…)
then one can order it again on e-spare-parts web site for a few dozens of pounds. It can be clipsed off and on again with a micro screwdriver on motherboard’s side (no lead solder…)