I have done what you're talking about and I have been extremely happy with the SeaGate STAE121 thunderbolt adaptor. I personally hook it up to a 1TB 2.5" HDD but you can plug *any* SATA 2.5" drive out there, which is the most common HDD out there. The SeaGate thunderbolt-SATA adaptor is only $70 bucks or so, but keep in mind that you also need to purchase the hard drive and thunderbolt cable separately.
Coolest of all, it supports SATA 6 and you can even boot to any hard drive connected in it. I've put a 512GB Samsung 840 Pro in it with a Windows 8.1 installation on it and I've been able to boot to it on my Macs and even my self-built hackintosh, and it's every bit as fast as an internal SSD the machine came with. It ran like a dream and was extremely easy to set up. Much much MUCH faster and more snappy than USB 3.0.
I cannot say enough good things about the SeaGate adaptor and in fact I'm so pleased with it that I throw SeaGate a bone and buy a SeaGate HDD whenever I am in the market for one.
There are two things to keep in mind though.
1. It's Thunderbolt 1, not thunderbolt 2. Thus it provides 10GB/s throughput and not 20GB/s. This has zero effect on the speed of the hard drive since SATA III only uses 6GB/s.
2. SATA drives are only guaranteed for a limited number of physical insertions, if I remember right it's 250 insertions for a given drive. This might become a problem if you swap drives all the time. My solution to this problem has simply been to get a much bigger HDD and to create several partitions on it with different formats when needed. This may sound complicated but it's extraordinarily easy, literally takes less than 5 min and a simple Google search will explain how to use Disk Utility to do it. I have one drive that I am pretty sure I have inserted into various SATA ports well over 500 times so I wouldn't worry about it just be gentle.
If you have any questions just quote this post so that I get a notification! Here's a picture of the adaptor:
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And here's a picture of it with an SSD. You can use any SATA hard drive not just SSD's:
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