You don't necessarily need a RAID card to run RAID.
RAID can be performed in software or hardware. Hardware RAID (which utilizes a decidated card/processor) typically has higher performance, and options such as parity), but you can do simple RAID such as mirroring and striping in software, with no RAID card required.
In any case, if I remember correctly, the iMac only supports a single hard drive, so RAID isn't an option anyway.
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