Why isn't my BBB booting from microSD card?

I have written a new Debian 9.3 image to a microSD card and have
plugged it into my BBB. However the BBB is still booting from the
EMMC. How do I make it boot from the microSD card?

Is there maybe something wrong with the microSD card image? It
automounts on the BBB OK and appears at /media/rootfs with what look
like the right sort of contents.

No there's something wrong with the version installed to the eMMC,
it's called being "ancient"..

Either hold down the "boot" button every time, nuke the eMMC, or
re-flash the eMMC..