I have this drive, seagate freeagent desk 500GB, make sure that your WBFS partition is the primary partition. I wish that I had a different HD. This is what I do to get mine to work, for some reason my drive must be turned off when I power up my Wii, I plug my Hd into a power strip with a switch on it.
1. If running USBLaoder from homebrew channel - first turn on your Wii, not your HD. Start the Homebrew channel, when HBC boots up then turn on your seagate. Now select Usbloader and it should work.
2. If running USBLoader from channel you installed on your menu - first turn on your Wii, not your HD. After the Wii menu shows then turn on your HD. Now you should be able to use the USBLoader channel.