You'd be storing everything on a SMC, and you can buy plug and play usb smc writers pretty cheaply. About £10 for a cheap mail order one.
A lot faster than doing it through the GP32, and you could use different types of card as well, such as Compact Flash which go up to far larger capacities than SMC cards.
But to be honest, I don't know why you wouldn't be using blank CDs for backup - far more cheaper and practical.