Thanks! I got it working now. Here's what I did:
I downloaded the latest navigator, firmware, and TFTP boot image mentioned here:http://buffalo.lithium.com/t5/Storage/F ... d-p/100319
I formatted the new hard disk in Win 7 using NTFS with everything in default.
I opened the NAS using instructions I found in Youtube (e.g., remove fan screw, pull out and unplug fan, insert pin in hole and then pry open enclosure, remove screws from HD, remove old HD), put in the new HD, and closed the unit.
Then, I used TFTP following instructions above.
Next, I installed new firmware following these instructions:http://buffalo.lithium.com/t5/Storage/F ... d-p/100317
The system timed out several times, but I just kept re-installing the firmware. After I rebooted the unit, the lights indicated two short and three yellow lights, which means the firmware is being reset. After a few minutes, a solid blue light appeared.
I forgot to add that the menu configurations were in Japanese, so I used the "translate to" option in Chrome to go to the language setting and use the correct language.