This leads to a related question.
Can anyone suggest reasonable quality USB SD card interfaces that work with the Raspberry Pi?
I suspect that almost every 32Gb class 10 card will work in external SD card slots, even if the system can't boot from them. This is naturally less than ideal, but given how little the USB SD card interfaces cost, it doesn't seem like a huge burden.
An ebay search for "USB SD Card", restricted to "buy it now", "Free shipping", and price of 0.00 to 1.00 returns 196 hits.
I've used some designed for full size SD cards, and many of them weren't mechanically robust. I expect that the ones for the micro SD cards would be more durable, but I haven't used them.
To me, "working well with the Raspberry Pi", means that I can plug it directly into one of the USB host slots, and still have access to the other slot. The ones I've played with, were too fat, until I snapped off the plastic case.
I know nothing of the I/O performance on these things.