Halving the speed may not be enough. I suggest 4x and absolutely never use more than 8x for CDs and 4x for DVDs. Higher speed burns always have problems, but the problems are usually hidden by the error correction code. From what I've seen I don't think Linux has error correction enabled during boot, thus you need a good clean error-free burn. And personally I want that error correction code to be able to help other discs over time recover from scratches and disc rot, so I always use low speed burns. At first I didn't want to believe this, but I learned the hard way when I couldn't recover stuff that I had burn "successfully" and verified at high speed years ago, now all of my burns are at low speed. It is well worth the few extra minutes to get a burn that will boot properly, or one that has a much better chance of surviving over time.