Compatibility with 2 gigabyte and larger SD cards has been poor, due to the SD/MMC protocol's using a 32-bit address field denominated in bytes. The SDHC standard addresses this limitation by using 32-bit block addresses instead. Both SD and SDHC are traditionally accessed as 512-byte blocks on 512-byte boundaries,
so the change to host software or firmware is minor but required. Before SDHC was standardized, various manufacturers "extended" the SD control block fields for their 2 GB and 4 GB cards in different ways. Those cards are incompatible with many SD and some SDHC devices, as they conform to neither standard. All SDHC readers work with standard SD cards.
If you're going to copy and paste wholesale from wikipedia at least say so, link to the article or put quotes around it rather than making it look like it's your own post. Or at least remove the cite links so it's not so obvious
[1]