Answer to: Only Hynix RAM compatible with iMac 21" late 2013?I have replaced the original ram with Corsair modules 2x8gb. No problems encountered.
Answer to: Is the Hard drive Upgradeable?I have successfully replaced the Apple supplied SATA HD with a Corsair m500 SATA SSD, using the parts described in the teardown for this model and one of the previous 21 inch guides. No special cable was needed. The original HD rubber cradle is glued to the sides of the drive. It can be removed using the spudger from this site and then sticked to the sides of the replacement sata drive without any new adhesive. The system fan is also still running at normal speed as before with the original HD from Apple (System has been up for hours with no increase in fan speed).