Nowaday, only HD 3"1/2 disk can be found at good old retailer's.
But or beloved CPC can't understand easily the concept of High Density Disk with 1,44MB avalaibleavailable...
So you have to cheat : just put some duct-tape (scotch-tape, whatever...) on the HD Hole.
Tada, a proper DD disk with 720 KB (if your OS allows it).Or a common 2x180KB (2x 178KB ?) with firmware Loco Locomotive Basic, if you have a manual side switcher button too of course).
PC users used to do the opposite : file a HD hole on DD disk...this worked well sometimes.
You can also modify your HD drive to behave as a DD one (but this would be permanent of course).. Check for appropriate jumpers on your drive!
== A clever choice ==