Solved: It turns out that OpenBSD won't attach sd/scsibus to sdmmc unless the card is formatted; once I formatted the new card with ExFAT, it was immediately picked up and everything worked. I have no idea what original state the card was shipped in.
I'm trying to install OpenBSD 5.9 from a bootable USB drive onto a new SanDisk 32GB SDHC card. Upon booting, I can see the following:
sdhc0 at pci0 dev 20 function 7 "AMD Bolton SD/MMC" rev 0x01: apic 4 int 16 sdmmc0 at sdhc0 sdmmc0: can't identify card sdmmc0: no functions
When I later run sysctl hw.disknames I don't see the SDHC card, presumably because scsibus never gets attached to sdmcc0. I'm assuming I should see "scsibusX at sdmcc0" followed by an "sdY at scsibusX" so the disk is made available. How might I go about troubleshooting this?