"seems my HDD is losing free space.. "
Yes, and the issue seems to be a lack of space. I'm saying that its a bad idea to try to store your data on DVDs now. They are more expensive than HDD, and less reliable.
This man tells true!
And if you REALY need to store data at DVDs take Verbatims! - Best of all I used.
But then, I spend tons of cash at cd few good years ago, and I want to trow them away but i dont have heart

I stoped buying DVDs/CDs. My friend had collection of about 5000 cd with movies few years ago. He burn them, watch few times,
one day he wanted to watch them once more - suprice - they were like 70% of those 5000 corupted thanks to localization he used to store them
- Sunny place near window - after 6m - 12m those cd got corupted

Sad but true story, I prefer to buy hdd, fill it with data, plug it out of pc (longer life) and store it in closet

And remamber YOU can rewrite Hdd any time you want!