The third element in a BBC hard disc system is the host adapter board. This one is made by Akhter and similar boards were also manufactured by Acorn for use in their own Winchester Discs and in the FileStore E20.
The purpose of the host adapter is to communicate between the BBC and the Adaptec disc controller board.
The 50-way connector goes to the Adaptec ACB4070 and the 34-way connector goes to the BBC 1MHz bus.
Some early SCSI drives can be connected straight into the host adapter board without the need for a separate disc controller. Apart from the Rodime RO652 used in the Acorn FileStore E20, I do not know of any discs which are suitable. If you know, please, please, please e-mail me!!!!