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I have a RAID 5 array of 3x1.5TB hard drives currently set up on my Asus P8P67 motherboard, and I'm wondering if I could add another 1.5TB hard drive to it without having to back up my data and recreate the array. I pretty much want to expand what I currently have. Is this functionality possible given a hardware RAID setup?

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The motherboards built-in RAID controller is usually not referred to as hardware RAID. In spite of the presence of a RAID controller in the hardware, all calculations are still performed by the CPU, making the onboard solution much more similar to software RAID than "real" hardware RAID. This is why some people call it fake RAID. Onboard RAID is a more neutral term. –  Dennis Jan 12 '12 at 2:44

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Yes, this is possible.

Adding an additional hard drive to a RAID array in order to increase the capacity is known as array expansion. One example of array expansion would be adding a fourth hard drive to a three-drive RAID 5 volume.

Intel® Rapid Storage Technology supports array expansion on platforms with the Intel® ICH10DO or Intel® PCH RAID controller hubs.

Source: Intel® Rapid Storage Technology - Adding drives to a RAID array

The built-in help of RST explains how to in the section called Adding drives to a RAID array.

However:

Since RAID 5 uses block-level striping, adding a drive to an existing array requires pretty much all blocks to be moved either within a drive or from one drive to another. Not only does this take quite some time, it is also dangerous. For example, a forced shutdown due to a power failure could cause you to lose all your data.

So, even though adding a drive to an array without backing all the data up is possible, I would advice against it.

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