RAID stands for Redundant Array of Inexpensive Disks.
Use a good RAID controller from a well known vendor. Then create a RAID 1, RAID 5 or RAID 6 array of 2, 3, or 4 disks (minimum) respectively. The disks have to be of the same size (usually) and can be a traditional hard drive or a solid state drive. In RAID 1 you create a mirror image of your drive on another same size drive. RAID 1 can also be created under many versions of Windows without a RAID controller. Some computer motherboards also allow you to create a RAID 1 array. For RAID 5 or 6, however, make sure that use a good RAID controller because without one, there is a high chance that your array can get corrupted and useless loosing access to the information stored in your array. In RAID 5, you create an array of 3 or more disks that not only gives you redundancy allowing loss of one drive without loss of data stored on the array, but also speeds up disk access but reading and writing to the 3 or more drives at the same time. You will have the capacity of 2 drives (in a 3 drive array) but loose 1 drive capacity for redundancy). For my servers and high end workstations, I have frequently used RAID 5 arrays of 3 disks using a good RAID controller. RAID 6 is an extension of RAID 5 where you will loose the capacity of 2 of the 4 drives to redundancy but can loose 2 drives and the array data remains intact. RAID 5 and 6 give you redundancy and speed. For mission critical servers (e.g. running a hospital electronic medical records), I would recommend using RAID 6 arrays.
My favorite RAID controller was 3-ware which was bought by LSI and which is now owned by Avago! They periodically (you set up the frequency) check your arrays’ integrity and rebuild it if necessary in case of drive errors. The controllers email me if there is an error with one of the arrays, allowing me to fix it in a timely fashion. I do not need to keep checking on them. If you intend to use the onboard cache on the RAID controller to speed up the drive access time, then make sure you get the battery backup for the cache which sometimes may cost the same as the controller! The battery backup is probably essential to prevent corruption of RAID array data during power failure even if you have a UPS. In my experience UPS’s occasionally fail when you need them.