I have a project at the moment that uses a PMOSFET to switch the supply to an SD card. According to the SanDisk SD datasheet available here:
Because there are clamping diodes on the CMD, CLK, and DAT0-3 pins, it is crucial to ensure that CLK, CMD, and DAT0-3 are at zero (0) volts during the delay and before the FET switch is turned on. If any I/O pin, (CMD, CLK, or DAT0-3) goes above zero volts during the delay and before power reaches the card VDD pin, it will forward bias the clamping diodes and can cause the card to go into an unknown state.
The card is connected to the SPI bus of the board which has several other devices using the bus. Since forcing the entire bus low indefinitely is out of the question, how would I isolate the IO pins of the SD card from the bus while it is switched off?
I am also considering switching the SD card supply rail over to an auxiliary LDO so it is no longer attached to the main supply (but I wouldn't imagine this making a difference to the answer).