I've read about other such mishaps, and I'm wondering if this could be
because the Beagleboard (and many SD card readers such as mine) aren't
UHS host devices and so presumably don't drop the voltage from 3.3v to
1.8v as the UHS-I spec calls for?
Hello Grant, I’ve been in testing the issues of classes for other projects not only BB boards, I found many cards sold in market class 10 but so bad quality and cheap, I recommend you to use this software http://crystalmark.info , it is the best tester and show you the real card performance. I will post a test on it soon.
I've read about other such mishaps, and I'm wondering if this could be
because the Beagleboard (and many SD card readers such as mine) aren't
UHS host devices and so presumably don't drop the voltage from 3.3v to
1.8v as the UHS-I spec calls for?
In order to comply with the SD card spec, all UHS cards must support
3.3 V signaling and power. If you see page 13 of the simplified SD
card physical spec version 3.01 [1], you can read more about this.