A RAID array goes offline when the hard disk I/O access fails. This RAID failure can result from a faulty RAID controller card, a server virus attack, a hard drive failure or multiple hard drive failure.
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If your RAID server failed and your RAID hard disks went offline, do not rebuild anything unless you are highly experienced in restoring from RAID failures.
This is especially dangerous if the RAID is attempting to rebuild the RAID array and you remove a RAID disk out of the RAID array.