Hi,
I don´t know if you have to format the HDD to EXT3 at first time.
If the router is start up then you have to open the Firmware Restoration tool. That comes with the CD in the box of your router. (Utilitys installation)
Then in the window you have to search for the Firmware maybe (WL700gE_1.8.7.8.nas) say upload and the software will be done it.
After the dos screen got away you have to wait a little time. The IDE LED will be flash a lot. After that finished you can look at the Internet Explorer with 192.168.1.1 if the window will open. Password and user name are
admin
If it is so you finished it off.
Have fun with your new HDD
Vazili