Use the official way to transfer files. It's in System Settings.

You can use checkpoint, it will allow you to easily create a backup file that can be restored and transfer it

I May Be Wrong But I Have Seen Transfer To Pc Option Works By Chooseing it Going Through The Prompts And Putting it in a computer and then taking it out and putting it in the new system but i could be wrong as 1 i saw it on a forum from 8 yr ago and 2 ive never done this

iirc your saves are hardware specific, meaning if you make a save on a 3ds, you can only use that save data on that one 3ds... or at least thats how super mario maker works-..