See the attached schematic for an SD Card module that is widely available.
As far as level shifting goes, it can be done simply with two resistors forming a potential divider, but this one has a single 10k resistor on each signal line acting as a default pull-up to 3v3.
How is this working? As far as I can see, those lines have a nominal 3v3 on via the pull-up, so when the uC is connected and a signal occurs, teh SD card line would either pull down hard to 0v, with 3v3 across the resistor, or be pulled up to 5v, with 1.7v across the resistor (5v-3v3; in opposite polarity).
What am I missing here, as it doesn't seem to do anything expect provide a default pull-up of the lines.
