|"When I connected temporarily a SATA disk on the MB plug, my two optical readers were not recognised any more. Does this mean that the MB will only accommodate a maximum of 6 physical disks?"|
No that means your MB can only support 3 controllers [ide1, ide2, promise]. There are limited irqs and base io address available. Engage another controller and someone is going to lose their irq/io address to the other controller.
The trick then becomes to disable everything in the bios not being used. if using USB for keyboard/mouse diable com ports and keyboard port. if not using builtin firewire disable it. See if there is a "reset nvram settings" or similar that you switch from "no" to "yes". This will force the mb to reread the resource allocations and perhaps assign them to your additional hardware.
Give a person a fish, they eat for a day. Suggest they internet search and they learn a skill for a lifetime.