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So I thought a raid could be made with 2 drives, is that not the case?
I have too many projects to take on another and learn how to troubleshoot a homemade NAS right now.
How come a 3 drive NAS is much better than a 2 drive NAS? Is it common for more than one drive to fail at once?
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RAID-0: All drives in the array (2, 3, 4, etc.) are striped together to make a single volume. No fault tolerance. If one drive fails, all data is lost. The benefit is added read/write speed.
RAID-1: Two drives mirror each other. If one fails the other one technically can be substituted. No added speed.
RAID-5: Multiple drives (2 or more) make up the useable space. Another drive is not counted in the useable space, but allows data to be rebuilt if any single drive fails. There is a performance hit (relative to RAID-0)
RAID-6: Same, but uses two 'parity' drives, thus fault tolerance extends to two simultaneous drive failures. Performance hit, but the added fault tolerance is highly beneficial.
There are additional RAID levels like RAID-50, which combine the benefits of RAID-5 and RAID-0, but these require a lot of drives and in most cases offer little price/performance benefit to the average user. RAID-2, RAID-3, and RAID-4 almost never used these days, for various reasons.
RAID-5 and RAID-6 performance increases with the number of drives in the array. I sell some RAID-6 SAN/NAS arrays with 16 drives (scalable to 80 drives.) Multi-user 4K/8K video editing houses use these.
For most users RAID-5 with four disks is the sweet spot. Good balance of cost/performance.