2 SD cards in parallel

@fat16lib: Can you explain this timing a bit? At what point does the Arduino become SPI slave, and the SD card become SPI master? If the SD card is always a slave, it needs to keep quiet until the SPI master tells it to talk.

Read the SD spec - it is impossible to access two SD cards in parallel on one SPI bus. Cards are not synchronous devices. They go ready at random times and return status at random times.

It is not as much the SD spec that concerns me here. It is the SPI spec. Does the SD card transmit with the Slave Select HIGH? I don't see an interrupt pin. It doesn't interfere with the w5100 on the same SPI bus. If what you imply is true, they should not be able to coexist. ??

Maybe it is just me, but if the SPI is not synchronous, what is the SCK pin for?

The SD card is always the slave but it controls timing through the protocol between it and the master. That's why you need to understand the spec.

For example when you send a command to the card it delays by holding MISO high until it is ready to send status. The master waits till it get something other than 0XFF.

The protocol is filled with stuff like this that allows the card to control the timing for all operations. That's why you can't enable two card at the same time and run them in parallel.

SD fat supports multiple cards but only by accessing one card at a time.

SD fat supports multiple cards but only by accessing one card at a time.

So you could possibly access files in both SD cards in one sketch? Just one at a time?

Correct. You can have files open on both cards and SdFat will handle it. See the TwoCards example in the SdFat/examples folder.

SdFat is designed to access any number of cards but each card has a 512 byte buffer. On a Mega or 1284 you could have a lot of cards with files open on all cards.