I think this was only true of the 21.5 models, not the 27" but I have to check ifixit's teardown report to be sure.
Looking at OWC, it seems the last iMac that they could reliably set up an internal RAID0 stripe was the 2011. I don't see mention of it for the 2012. hmmm. While it has two SATA ports, maybe you also need two exactly identical drives? Maybe they're not available in a mSATA/SATA pair?
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