be aware that you would need a Thunderbolt compatible enclosure to do exactly as you want.
It would be much easier, and cheaper, to find a USB enclosure and use that
I am getting a 128 gb ssd from Crucial to use as my main drive in my late 2011, 13 inch mbp . I was wondering if I could take out my normal hdd and use it through thunderbolt with an external enclosure I found as a secondary data drive. I would just get a 256 gb ssd, but I cant afford it. Thanks, stelly.