There are differences in the different generation of SD cards,  these different generations are not backwards compatible with legacy devices.  

For Example:

If you have a Device that will read SD cards this device will read up to 2 GB SD cards and will not read anything higher. 

if you have a Device that reads SDHC cards this device will read up to 32 GB SD cards but will not read anything higher.

If you have a Device that reads SDXC cards this device will read up to 2 TB Cards (theoretical limit, current maximum is 1 TB)