1. Ensure your 500GB drive is formatted using FAT32 as mentioned previously and test this first by connecting it to your PS3 to find out if it sees it.
2. Use backup manager (or any file manager on your ps3) to copy the game to your external usb. Or, if you have the game available in your PC/Laptop, connect your 500GB drive, create a GAMEZ folder and copy the game into it.
3. Once you verify your game is loading from external, you can now delete it from your internal if you prefer.
Note: To format your 500gb drive to FAT32, there are tools available that were posted by users in this site.