On the Shugart FDD interface pin 2 is "disk change detect", my understanding is the drive generates this signal to inform the host.

In the Yamaha C1 Music Computer, this signal is routed to an 'IO Controller' listed as 'YCAI' - presumably some customer logic or gate array of some description - which also deals with disk controller address decode and some other stuff.

But, the floppy drive disk change signal also turns up on the Hard Disk Interface. I cannot think why.

A couple of people (eg here) have found that the machine cannot boot from floppy without the HDD board connected. As I see it this could be because of one of three reasons:

1. They didn't change the position of the HDD switch (SW2) when removing the HDD board
2. They didn't set the HDD device type to 'Not installed', or
3. Some electrical reason related to the signal.

Any thoughts on this greatly appreciated.