imac 21.5 Core i3 mid 2010 - how many SATA ports available?
That model does not support SSD and HDD in addition to the optical


https://everymac.com/systems/apple/imac/specs/imac-core-i3-3.06-21-inch-aluminum-mid-2010-specs.html


The 27” from 2010 does, however. It just needs a mounting bracket and the SSD. It’s SATA 3 Gbps bus and this link has lots of upgrade referrals:


https://everymac.com/systems/apple/imac/specs/imac-core-i5-2.8-27-inch-aluminum-mid-2010-specs.html


You’d want to get a hybrid SSD/HDD drive if you needed size and space on the 21” unfortunately.

Your mac would need to move the Optical external and put the SSD in place of the optical slot if you chose to retain a full size HDD.


https://www.ifixit.com/Guide/Installing+iMac+Intel+21.5-Inch+EMC+2389+Dual+Drive/8643