Many Modern CPC users replace their old 3" with an external 3"1/2, often adding a Disk drive A-B / B-A switcher (allowing the use of an external disk Drive as if it were the internal one = Drive A) and/or a side switcher to allow the use of a 3"1/2 disk like a 3" disk... switching manually the sides as needed by good old 3" disk drives (yet a decent software can do it).
The side switcher and A-B drive's switchers are needed only if you use old software (using [[AMSDOS|AMSDOS]]), as most of them couldn't really figure they were loaded from B drives, or had no such feature as double sided drives.
They were designed for good old 3" drive so the 720KB DD external 3"1/2 is not implemented.