My laptop kept freezing so I thought I would clean it. Following online instructions, I opened the casing & cleaned the fan & electronics with compressed air. After reassembling, the laptop wouldn't get past the grey page with the apple & spinning wheel (that usually appears briefly when starting up) I tried starting with start up disk to do a disc repair. Mac HD (499GB) seemed ok, however there was another 501GB drive which was in red writing. I only have one drive installed on my Mac, so is Mac HD a part of this greater drive? If so, why is Mac HD ok but the greater drive faulty? Any tips for what to do next?