Thanks for sharing about the power off bit... I don't know for sure if thats really the solution but I had the exact same problem tonight and I just got it to work finally by shutting down the xbox and FlashFXP and then turning the xbox back on and FTP'g to it. I think you could get it to list the other drives that evox showed by editing the .xml file just copy and past the permissions and change the drive letter to the missing xbox letters.
Peace