First of all, there is no way of doing this without loosing the existing data on the unit.
1. Create one large NTFS partition on each of the new HDD's and insert them into the Linkstation, the error LED will start flashing 6 times.
2. Download the TFTP image for your unit from here: ftp://24.153.165.234/array1/share/nas-recovery/TFTP%20Boot%20Recovery%20LS-WXL%201.33.exe? and the firmware from here: http://www.buffalotech.com/support/getfile/ls_series-154.zip?, you will also need the Nas Navigator software from here: http://www.buffalo-technology.com/files/downloads/NASNavigator2.zip?
3. Connect the Linkstation directly to the PC/laptop via the LAN cable and turn off all anti virus and firewall software. Set the IP address of the LAN card on the PC to be 192.168.11.1 and subnet of 255.255.255.0
4. Unzip the TFTP image and run the TFTPboot.exe file, once the black screen appears and is stuck at "accepting request" press and hold the function button on the unit for around 10 to 15 seconds. Once this is done you will see two sets of blocks passed.
5. Wait 2 - 3 minutes and run the Nas Navigator, it will as you to automatically correct the IP address, click NO, you should see the unit in EM mode and the IP address will be something like 169.254.23.49
Set the IP address on the PC to be 169.254.23.50 (make sure the first 3 sets of number are the smae but increment the last one by one digit)
6. Unzip the firmware and double click the LSUpdater.exe file and it should find the linkstation. Click Update and this should install the firmware for you.
NB. If during the firmware update you get a message like "Could not confirm weither partition exists" open the LSUpdater.ini file and change NoFormatting = 1 to NoFormatting = 0 , close and save the file, run the LSUpater.exe file again, right click on the title bar and select debug, make sure all the options on the left and right hand side are selected and proceed with the update again.
Let me know how you get on :-)