First backup the computer and save all data.

Then, get into the BIOS and delete the RAID setup. Install the new identical HDD in place of the defective HDD. Then re-setup RAID just the way you set it up in the first place. Sometimes, just replacing the bad HDD and installing a new HDD will rebuild the RAID.
 

tokencode

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If you try the bove suggestion, DO NOT initialize the array, some controllers will do this automatically. A safer and more time-consuming way is to make images of each drive as the exist, use a recovery utility such as R-studio to reconstruct the RAID from those images and copy thedata to a new, good partition.