This might not be the right forum for this, but I like this forum so we'll see how it goes. The same way that Apple signs versions of iOS that later prevents downgrading devices unless the version is still being signed seems to be where they're going with OS X now. I might be wrong, but at least with the new macbook (and where they seem to be going with iOS devices), that target disk mode might soon become a thing of the past. I know some Macbooks can't do it due to lack of firewire/thunderbolt ports and as long as they're making MacBook Pros with TB ports then it might hold on. However, if they go to USB-C as a standard (which is a reasonable assumption) then we might get to the point where once you've upgraded to a newer version of OS X that you might not be able to downgrade. What do you think?