Could you check what the spec's are for your Sandisc 64GB card? (read/write speed)
It really sounds almost unbelievable, that there should be such a difference, but on the other hand I just read that the maker of the Altmann Tera Player said that he observed jitter directly from flash memory, which might be the difference in what you are hearing. I expect that if this is true, then it's because a very high read speed card will produce less jitter.
24-bit / 96kHz FLAC files are only about 2MB/sec, so in theory this shouldn't be the issue but then that's just theory
I also think the only reasonable explanation is related to some jitter in reading the card. Reading speed should be then the key parameter, but also the amount of memory on the card depending upon the way the memory is accessed. Naively, I would expect the firmware to play a role and perhaps issues can be fixed.
I have just checked and the Sandisk class 10 is rated 30 MB/s (they don't specify separate read and write speeds), while the Toshiba Exceria seems to be the fastest available at 95 MB/s read speed (Samsung Pro is rated 70 MB/s).
Edited by rmap - 6/14/13 at 8:40pm