Why can I power an SD card reader from a digital pin?
I just tested a SD card in an adapter (with level shifters). Without any activity the SD card is only drawing around 0.5 mA. So that is in the capability of an output pin.

Doing a directory list appears to use a maximum of 7 mA, so that is OK.

Writing a file used a maximum of 54.3 mA, which is exceeding the absolute maximum ratings for an output pin (40 mA - recommended only 20 mA for continuous current).

Now you may say that "it works" but the question is: for how long? Exceeding absolute maximum ratings is a path to damaging your processor.

If you want to turn the SD card on and off from time to time, to reduce power, I suggest you arrange to switch the 5V line (and not use a digital output). For example from this page about a low-power temperature sensor I have this schematic:



The MOSFET there switches a 5V line on and off under program control.

That particular design includes a SD card, and that particular gadget has been working for a couple of years now.

Photo of hardware: