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I found a refurbished Mac Mini which I set up for my two grandsons, ages 13 and 16. Young people, being a bit impatient, I decided to add a 240Gb SSD as a startup disk and use the original HD for Time Machine backups and less critical data.
I ordered the "Mac Mini dual drive kit" (including tools) from iFixit and a 240 Gb SSD from Crucial. I followed iFixit's "Installing Mac Mini Late 2012 Dual Hard Drive Kit". Duck Soup. Couldn't have been better. I formatted the new SSD, Restored a bootable clone on it, set it as the startup drive and that was it!
Advice: choose the right Guide and read/view it thoroughly. Read it again ... until you can visualize each step. Get the right tools - if a tool is mentioned in the Guide, have it at hand. Don't work to a deadline. For instance this task took less than an hour, but I didn't count on it and did it when I had plenty of slack time.