What is the device name of the SD card? I'm curious why there isn't a /dev/sdb.
The automounting is handled by udev. When your device is mounted, find out which device it is. If it is /dev/sda2 or /dev/sdc1, add the noauto option since it is a removable device.
When you plug in a removable device, normally a mount point is created on the fly under the /media directory. Sometimes it is given the name of the mounted device such as "/media/SanDisk".
There is a Redhat Magazine link explaining udev: