Unfortunately it seems you are a victim of a HACKED player. This is where the seller or manufacturer has 'hacked' the player to display more memory than what is actually installed on it. In your case, you only have a 512MB memory chip inside, but it has been hacked to show 8GB instead.
Where and who did you buy it from? I would strongly suggest you contact the seller and ask for a refund or exchange (though partial refund seems to be the best solution if the seller is overseas).