Pretend I want to buy addictional HDD. My current HDD is internal 160gb 7200rpm 2mb cache. If I buy extrenal HDD 7200rpm 8mb cache... Will it add up to my current internal HDD? and do I have to wipe everything on my current HDD? My current HDD is internal with 2mb cache... can I add additional external HDD 8mb cache to add up together even if the cache size is different? Will the HDD add up together? or I must replace it?