I just bought a 750GB Western Digital My Book external hard drive to use for Time Machine backups and other storage. The problem is, when my Macbook is writing to the drive it sometimes makes a weird scratching noise. Is it okay for a hard drive to make noises when it is writing data? Also, when the hard drive is plugged in to my laptop, even when inactive, the two lights on the front flash. Does this indicate that something is wrong?