I believe that I had the same concern. After rebooting an upgraded system (uptime of 6+ months), my SD card didn't appear in /dev/sd* .
lsusb showed the card reader:
Bus 003 Device 004: ID 0bda:0129 Realtek Semiconductor Corp. RTS5129 Card Reader Controller
My dmesg output looked like:
[ 1090.387710] mmc0: card 59b4 removed
[ 1097.802651] mmc0: new ultra high speed SDR50 SDXC card at address 59b4
[ 1097.802874] mmcblk0: mmc0:59b4 SD64G 59.6 GiB
[ 1097.804711] mmcblk0: p1
After an hour or more of poking around the web, it appears that the device is now exposed in /dev/mmc*
# ls /dev/mmc*
/dev/mmcblk0 /dev/mmcblk0p1
With that new knowledge, I again attempted to mount...
# mount /dev/mmcblk0p1 /media/usbSD/
#
Huzzah. Works great.