Thanks crispyoz. I tried a few different PSUs and reflashed the firmware, but no luck. I ordered a couple more so hopefully I'll have better luck with those.

@William-Allen 2.4A Micro USB supply made no difference. Not sure if the input is current-limited somehow. Also, USB thumb drive works fine, and I'm guessing that draws more current (of course not through the MCU...)

@crispyoz Thanks for the response!

Yes, an expansion board, using the latest firmware. I already reseated the controller after inspecting the SD slot for any obvious signs of damage.

I'll look into the power issue. If I connect, say, a 2.4A charger cable to the microUSB slot of the expansion board, should that provide sufficient current, or should I feed the MCU v_in pin directly from a power supply?

I've tried 2 different cards now on my Omega2+, (8GB and 32GB), and whenever I insert either, I get a bunch of errors about failure to read blocks. They're both formatted ext4, and both work without issue in other devices. Is it possible my Omega2+ has a faulty SD reader, or is this a common problem? I know they're inexpensive, but if this is a known issue, I'd prefer not to throw good money after bad.