Well, this issue has been solved. I simply updated MSI motherboard BIOS. A 10.4 release was installed and I updated it to 10.9, downloaded from MSI Support Web page. Actually, the update reset BIOS settings to default, so I'm not sure if the "visibility" of the HDD is due to the new BIOS or to the return to default settings. Maybe the PC vendor had set some particular configuration, in order to avoid installation of additional drives. Anyway, now I have two HDDs in my PC and it's OK.