Originally Posted by dwig
Physically, they are identical.
They are quite different in terms of their logic. Readers that aren't specifically designed to handle SDHC cards (whether standard or Micro) can't read them reliably, if at all. The same applies to SDXC cards, which are the newest type of large capacity cards in the SD family.
That is a nice general description, but the question was specific to the Nook.
The Nook can handle SD and SDHC, so that should cover the specifics of the question. AFAIK the Nook can not handle SDXC, which is what is necessary to go over 32 GigaBytes.