Raid is the combining of different harddrives to act as a single unit. The drives are matched units so speed and all is alike. now think about a stereo system and how music is channeled to left and right and front and back and so on. In a raid array(what it is called) the data is written in segments to each drive. this speeds the system up as each drive only has to search for a part of the data instaed of all of it. In a 2 disc array..data is written and retrieved ...50% on each disk so it actually speeds up the system twice as fast. a 3 disc array would write and retrieve 33.3% for each disc and speeds up the sys that much more and so on. The down side of that is that if one disc goes out then you have lost all the data because the information on the other disc is useless. There are certain arrays to combat this and it becomes safer and the data is easily retrieved.