The FDC controller inside the CPC or in the [[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. And AMSDOS supports a maximum of two disk drives only.
However, the FDC in the [[Vortex Disc Drives|Vortex disc interface]] does have the US1 pin connected and its VDOS can manage up to 4 floppy drives.
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