Thanks for the reply. The RF24 library does not allow me to specify the MISO and MOSI pins unfortunately. The SD library is the standard Arduino library v 1.2.4. I have not been able to figure out if they are using software or hardware SPI. Not sure where I would find that without really digging into the source code. I do have an oscilloscope so I may try to debug the bus. I will also order some tri-state buffers. Thanks for your help.