Hi Dave2Satellite - I read your thread with great interest as I am in the same position. In my case I'm using the SD library with an A/D on the SPI line - an LTC1859CG#PBF to be precise. The SD card works fine but the A/D returns all 1's. I placed a data analyzer on the lines and it appears like the SD card is sending garbage on the MISO line after the file is closed and the A/D interprets the garbage as 1's. The A/D still doesn't work properly when I upload a sketch that contains only the A/D and SPI - without the SD stuff. I have to remove power to the arduino to get the A/D working properly by itself with no SD references. Also - if I remove the SD card while the A/D is reading garbage then the A/D starts working magically (the MISO line settles back to normal operation). I went back and tried to implement the SdFat library but it didn't work - same results. You mentioned you only made a few modifications - were any of them to the SdFat library by chance? Any help is greatly appreciated! - -Leah