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Redundant Array of Independent Disks. This is a technique that server-class computers use to both improve speed of on-disk storage and to improve reliability. Using multiple disks, the computer splits up and/or copies data across the disks so that if one of the disks breaks, no data is lost and the server doesn't crash. You will often hear storage described as RAID 0, RAID 1, etc. The number indicates the type of storage, not the number of hard drives. A good resource for learning more about RAID storage is on Wikipedia.
In hardware, RTU is short for Remote Terminal Unit. In SCADA systems, an RTU is a device installed at a remote location that collects data, codes the data into a format that is transmittable and transmits the data back to a central station, or master.
In Modbus devices, you will sometimes see "RTU over TCP" as the connection type. This just means that TCP is used to transport data through the network, but the data being transported is using Modbus' RTU format.