What's been said is basically it no matter where you're from
There's nothing illegal using Flashcard for 'Software development' (i.e. Homebrew) but it's totally illegal to make a copy (even if you own the original game cartridge) & then use the copy (ROM) on a flashcard
It is a little bit of a grey area about 'making a copy' of your commercial game cartridge - with normal software (PC/MAC etc) sometimes the license allows you to make a single copy of the software for backup purposes & some people believe this also applies to console games
AFAIK this is totally illegal. The only way to confirm this is to read the license agreement that comes with the game/software (which in some cases can be many pages long) which most people cannot be bothered to read & just click [Continue] to bypass it
I believe the 'making a copy of your save games' is allowed though