Can someone help me in making a decision. I am trying out one thing - to encrypt the data that is written to

external USB storage device ONLY. I think I have 2 options,

1) To make use of FiST and develop a stackable Filesystem that can extend the functionality of read/write command and introduce encryption/decryption there. basically a filesystem filter driver.

2) I thought I can modify the USB device driver(since I am worried only about the exernal USB device) and modify the read/write method to introduce the additional functionality of encryption.

I am not sure which is the best/possible way and also curious to know the reason for the same. Please help