Is your Mac running with Mac OS X ? It seems it is supposed to propose the option to format in Fat32 even if the card is > 32 Gio.
However, Windows does not give the option to format in Fat32 for > 32 Gio cards.
(Gio and not Go according to Wikipedia. Please see Wikipedia's article on Octen (Computing), section "Unit multiples", but I believe in our case it does not make such a difference)
Maybe is it linked to SDXC ?
Anyway, have you tried to use guiformat like stated previously ? It can force Fat32 on SDXC cards. Your computer may find it is a problem but you can just ignore it and copy your files, it will be ok.
maybe there's a problem with your micro SD?