A friend and I had a debate a long time ago, regarding DVD Vs portable HDD. At that time, I was in favour of storing data on DVD.
Now, I have changed my opinion to a certain extent. I have found that while storing data on DVDs is cheap, reading and writing the data is time consuming. Additionally, is the storage. To add to that is the fact that DVDs sometime go bad and are not readable, whoever nicely you store them.
In case of HDD, the larger the HDD, the bigger the chances of losing lot of data in case of a HDD failure. But data access is much faster.
So, for data that you need immediately should be stored on HDD. For important data that cannot be downloaded or recreated, store an additional copy on a DVD for safety.