The fact that you had one card work intermittently makes me wonder if this is an issue related to the level shifter...
As
@tashtari knows, I've been having my own fun'n'games getting my TT(rev 1.0)s up and running.
After initially having one work with my Plus, I started experiencing flakiness and flat out refusal to work on any of my other Macs. Fortunately, I have a couple of real HD20s (+ a couple of FloppyEMUs) so knew the issues weren't with the Macs.
After much head scratching, I located a couple more uSD cards + an old SanDisk USB/uSD adapter (was using a Lexar).
I now have all my TTs working - both with the previously flaky (2gb) uSD as well as another 4gb I had been using on my rPi. Both were imaged using the SanDisk adapter.
I can't say for sure the Lexar adapter was causing the problems, I can only share what I've found. Of note - I've had zero luck with the other uSD (a 8gb no-name) - no matter what USB adapter I try.
Perhaps the SparkFun s marginal, or, maybe some uSDs suck more juice than others exacerbating the situation? I have some more uSDs on the way, so we'll see how it goes with those....