How can I decrypt an encrypted SD card (HTC Evo 4G) on a different device?
The simple answer is: You Can't

The volume is already encrypted and can only be read as random hex because the required key is stored in your phone in order to decipher it. Unfortunately there is no way for you to recover your sdcard contents because the key on your phone is also lost when you factory reset.

Just reformat it if you still want to use your sdcard.