I just wanted to share my recent experience that might help others: I recently received a BeagleBone Black rev C made by Element14 (I ordered mine from logic supply FWIW). The first thing I did was create a Volumio image on a microSD I already had, but I could not get my BBB to boot off it!
The BBB has three buttons. Two are close together, adjacent to the four LEDs:
S1 - Reset
S3 - Power
And on the other end of the board is:
S2 - Boot
Everything I read said to hold down S2/Boot while connecting power to the BBB, wait until the user LEDs light up, then release power. I could not boot from MicroSD using this technique. I tried two different Volumio versions on two different MicroSD cards. (Side note: USR1 LED means MicroSD card access, and during all this, I never saw that LED light. But I saw lots of activity on USR3, which means eMMC access.)
Trying the same thing over and over again without any new result is madness, so I took a different approach. I instead held down S1/Reset while applying power. When I did this, the single power LED came on right away. But as long as I held down S1/Reset, none of the four LEDs lit up (and I kept holding for a good minute). But shortly after I released the S1/Reset button, all four LEDs light up simultaneously for about a second, then they all started flashing in such a way as to suggest the system was booting. But this time, I saw virtually no USR3 LED (eMMC) activity, but lots of USR1 LED (MicroSD) activity. And indeed, after connecting an ethernet cable, I was able to login to Volumio.
TL;DR - can’t boot MicroSD via S2/Boot button? Try S1/Reset instead.