iMac 21.5” mid-2010 SATA Specification
Your systems SATA port is only SATA II (3.0 Gb/s) . So It Will Not Support . For Details :- 


  https://www.ifixit.com/Answers/View/454939/Can+SATA+III+HDD+be+installed+on+mid+2010+iMac+Intel+21.5%22