My combo drive is acting up. It seems to get stuck and produce a read error on 90 per cent of discs I put in it. It stops about 15 - 20 minutes into watching a DVD and cannot proceed further once it does this. That said, how long do apple techs normally take to replace a combo drive? Also, am I better off going to the Apple store, or just sending it off to Apple to fix? My MacBook is under warranty.