The FDC controller in the CPC or [[Amstrad External Disk Drive|Amstrad DDI-1]] expansion doesn't have the US1 pin connected. This means it can only select floppy drives 0 and 1, and not drives 2 and 3.
However, the FDC in the [[Vortex Disc Drives|Vortex disc interface ]] does have the US1 pin connected, so it can manage up to 4 floppy drives.
<br>