So... It turns out that my iMac is not reading any DVD. It will perfectly work with audio CDs, but when I insert a DVD the crappy SuperDrive starts to read, then it stops and finally will eject the disc. I tried to clean the drive using a credit card and a soft clothe, but still it's not working. Any idea? Going the technical support and loose my iMac for a relatively long period of time is my last resource, so I really want to solve this fast and by myself.