I'm not certain this is the culprit, but the overall consensus that I've read online points to the fact that most (or all?) available external Thunderbolt drive enclosures that are *bus-powered only* (so no independent power brick with the enclosure) have difficulty providing enough power to SSD's that exceed ~240GB. And without enough power the system freezes or causes other errors.
Here's at least one thorough examination that came to this conclusion:
The solution, from what I've read at least, is to find a Thunderbolt adapter with an independent power source or use a smaller SSD (under 240GB it seems) that is capable of running on bus-power only.
I'm looking into doing the same as you (using external SSD as primary boot drive) but there aren't that many relatively-affordable enclosures to choose from at this point (off the top of my head just the Seagate GoFlex, Buffalo Ministation, and Lacie offerings come to mind) and there's still an outstanding question as to whether the available enclosures support the SATA III spec to really get the most out of the fastest SSD's available today.
Of note, I believe this version of Seagate's adapter (the "Desktop" version) does come with its own power brick and could solve your problem, at least according to the article I linked above.
Hope this helps, good luck!
Thanks BinaryMango. I didn't even think power can be the problem.
Already owning this Samsung SSD I think I'll purchase the Seagate desktop version, which I previously discarded for its size, the higher price and because isn't bus-powered. If only I had known!
there's still an outstanding question as to whether the available enclosures support the SATA III spec to really get the most out of the fastest SSD's available today.
I think Seagate Goflex Thunderbolt supports SATA III