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 sofware software can do it).
The side switcher and A-B drive's switchers are needed only if you use old software(using AMSDOS), as most of them couldn't really figure they were loaded from B driversdrives, 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.
Orion Prime uses the Double side feature, enabling a simple 720KB disk with no manual side switches.
Rick Dangerous 128+ (1.1 addonadd-on) seems to allow the game to be loaded from the B drive.
It is up to the CPC-scene to design their softwares software to include those options, allowing more un-modded drives to be simply used as external B drives with no need to add extra buttons and cable assemblies on the Amstrad.
As modifying all the software library of the good old times seems impractical, yet modern era software have to use this.