Do you want to get back your erased data back or just fix the capacity problem？
Actually, in my opinion, these two questions are not difficult.
You could firstly recover your data at the aid of a free drive recovery tool and format this drive to fix the capacity problem afterwards. (In fact, I have ever tried this freeware to recover my data after accidental deletion. The result is great. You could experience it after applying it personally. ) Note for you:
1. Stop using this drive immediately to avoid overwriting your original data.
2. Save your recovered data on a different drive in case of recovery failure.
3. Not forget to format this drive.
4. Back up your data regularly in the future.