Gracias por vuestras respuestas. Ya he salido de dudas. Todo esto viene al comentario que lei, que os copio aqui:
The discs that were easily scratched at the top were Compact Discs. The actual data was basically an extremely thin layer "painted" on the top of disc. If you scratched it there, the data was gone.

SD DVD (and HD DVD) on the other hand are different. They are made with 2 pieces of plastic, and the data is in the middle. You can scratch the top side with no ill effect. If you scratch the bottom, it interferes with laser reading the disc, but if it's not too bad the player can read past it (or make up for it w/ digital error correction).

Blu-ray is like an upside-down CD. The data layer is extremely close to the bottom of the disc (side w/o the label). Blu-ray was so fragile though, that they developed a physical coating that goes over the data layer. The coating is much harder than normal disc plastic, and quite difficult to scratch. But if you do scratch it (which is possible) you'll likely tear right into the data layer. (Which is why it's implausible that BD's will ever be "repairable" like DVDs & CD.)