This signal differs between floppy drives model:
*For 3inch floppy drives (pin26), it is a "Ready" signal(pin 26). The /RDY signal turns on when a disk is inserted and spinning in the drive, usually after detecting 2 or 3 index pulses, depending on the drive model. It turns off immediately if the disk is removed or the motor stops.
*For PC 3.5inch floppy drives (pin34), it is a "Disk Changed" signal(pin 34). The /DSKCHG signal determines whether the same disk loaded during the previous disk access is still in the drive.
The easiest solution to this issue is to force the Ready signal on the cable. However, the CPC will hang if you type the CAT command with no disk inserted (inserting a disk will unblock it).