booting from TFTP is one way to get the device into EM mode so that you can install the firmware. For this particular model you should be able to just insert 4 blank drives and then power it on, the device should eventually boot inot EM mode from it's internal Flash (it might take like ~15 minutes total).
Once you get the device into EM mode you can install the firmware using this process (starting at the part where you modify tsupdater):
https://buffalonas.miraheze.org/wiki/Restoring_Stock_Firmware_without_TFTPIf you are able to boot into the firmware from an existing disk you should be able to skip all that by doing the following:
1. boot from just the working drive
2. insert one of the new drives, ensure it is completely blank
3. format it from the web interface (this will also install the required boot files)
4. shutdown the device and move the new drive to bay 1 and restart
5. assuming it boots you should be able to add the other blank drives and create arrays as needed.
the most obvious reasons I could see for it failing would be that the new drives aren't blank or that those newer drives aren't compatible with this older device. I doubt the drives wouldn't work but it might be worth testing the procedure on drives less likely to have compatibility problems if the above methods still aren't working.