Unable to rebuild in mirrored RAID after hard drive failure

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I plan on getting an external mirrored raid setup for my backups of all my work that I have done. I am currently testing out how the built in software RAID works in OSX. I have setup a mirrored RAID with two flash drives and it works fine, i then tried to simulate if one of the disks were to fail by reformatting it, and then i tried to plug it back in to rebuild it, but it does not allow me to do so the rebuild button is greyed out. Could it be because i had the RAID setup as automatically rebuild? If someone could walk me through the process of rebuilding the raid setup that would be great, i would like to figure this out before purchasing the harddrives.