Ok, so what you need is to have a USB to SD card adapter coming from the PC, and the 5v/3.3V SD card adapter coming from the arduino, with the control signals switched.
I would get a USB to SD card adapter like thishttp://www.amazon.com/Datafab-KESD-USBA-Reader-Extension-Cable/dp/B002ZVE4HC
open the case, and wire from that connector to a buffer chip that does 3.3V signals, and wire the OE/ signal so its under Arduino control.
Then wire the Arduino SPI lines thru a 5V to 3.3V level converter like this, and wire the OE/ signal so its under Arduino control.
I have used 74AC125D, not the HC that is shown here, this is just what I happened to find first.