I found my earlier post: http://arduino.cc/forum/index.php/topic,79551.0.html#msg682829
Mr. fat16lib shares some good information:
I am amazed your setup works at all. You need to clean up your wiring so the SD is close to the CPU and use very short wires for SPI.
The version of SD.h included with 0022 opens files with the O_SYNC flag so there is an incredible amount of I/O to the card.
Also opening and closing the file for each data point causes more I/O as the file grows since open does a seek to the end-of-file.
I/O to the card is done in 512 byte blocks and a sync call requires at least 2048 byte of I/O. I suspect you are doing over 50,000 bytes of I/O to the SD for each data point.
Now I must puzzle out the meaning of:
I suspect you are doing over 50,000 bytes of I/O to the SD for each data point.