You might want to check out this page on the GD-ROM using an Oak IC and IDE:
http://mozcom.com/~sknkwrks/gdcontroller.html

Michael Robinson

I know this. I'm sorry that I wasn't more specific in my question. It leaded
to answers I did not wanted to hear ;) .

 I know we can use the expansion port to hook-up a IDE / ISA interface, as
BitMaster32 did. I plan to make such a board in the upcoming days. This can
be uses to attach wd* and/or ATAPI devices.

But my question was specifically directed to the connector that connects the
GD-ROM drive to the motherboard, inside the dreamcast. I ask what kind of
bus it is. Here I put pictures on my gate so you can see.

GD-ROM unit. http://neutr1n0x.no-ip.org/dc/GD-ROM.JPG
connector on the GD-ROM. [Top right]
http://neutr1n0x.no-ip.org/dc/gd-rom-sw1.jpg
connector on the Motherboard. http://neutr1n0x.no-ip.org/dc/MB-ports.jpg

Where can I get the connector's pinouts and if it is possible to use another
storage device instead of the GD-ROM plugged into that particular port.
Again light me up and I'll be happy :)