Hello I'm thinking of buying a 27' retina iMac. I would like to use my existing 21.5 mid 2010 iMac as a dual display. can this bee done? What cable would i need to do this? Also should I transfer my existing data to the new iMac first?
The 2010 21.5 inch iMacs cannot be used as external displays.
A simple way to migrate your data would be to backup your existing iMac to an external hard drive using Time Machine. Then, use migration assistant to restore your data during the setup process of the new retina iMac.