No, you can not 'over-write' it, you first have to erase what's on the disc.
Go to 'Applications'
Choose 'Disk Utility' (Not I-Disk Utility!)
Then you'll see the name you previously gave the disc in the left top corner. Highlight that, and opt for 'erase' in the menu where it says 'First Aid', 'erase', etc.. After that's done your disk will be empty again.