I got asked very often, how to connect an extarnal disc drive to a CPC. It is very easy to build a data cable. Use a flat cable and special connectors to crimp. In my drawing I use the pin description printed directly on the connectors. Please ignore the pin description in your CPC manual.
Data cable connection diagram
The pin 1 of the floppy connector must be routed to pin 2 of the CPC connector. All other cables are parallel. Some 3.5 inch drives need to be shorted at the READY signal (pin 33) with ground (pin 34). You can do this directly on the floppy drives back plate or with the cable, like shown.
New PC drives are mostly jumpered as drive B:. In that case you do not need to jumper anything.
Now your new external disc drive needs power. You may get it directly from the 12 V plug in you CPC monitor. A DC regulator, like in the next picture provides the 5 V. That's all.