To decrypt the files on your storage card, you need to use a slightly longer method. You must copy over the files from your memory card to your computer using Windows mobile device centre in My Computer or ActiveSync. You must then copy the files back to your memory card after disabling encryption.
To disable encryption on your memory card carry out the following steps -
Step 1 : Make sure the Memory card is inserted into your device.
Step 2 : Go to the Start menu and select Settings -> System Tab -> Encryption.
Step 3 : Remove the check from the option Encrypt files placed on the storage card.
Step 4 : Select the OK button to confirm the settings.
Any encrypted files that were not transferred to your PC will be unusable after encryption is disabled. A hard reboot will erase the decryption key present on your phone so remember to back up your files before doing it.