There is nice explanation here http://www.hjreggel.net/cardspeed/special-sd.html The 2GB limit is because of FAT16 limitation being part of SD specs, not on lower level (i.e differencies between cards). Problems on lower level start already over 1GB. 4GB is no worse than 2GB on low level. So it does not vary by card manufacturer how it is done but by 'reader' manufacturer how it is read.