As jotin says, !!THIS WILL VOID ANY REMAINING WARRANTY ON YOUR DEVICE, PROCEED AT YOUR OWN RISK!!
OK, now that that's out of the way, i upgraded my TS-HTGL/R5 at the weekend from 2TB to 8TB.
Here's what to do.
Format all the HDD's you want to insert into the Terastation (one large NTFS partition on each HDD is fine)
Insert them into the Terastation and after a minute it will display E06 TFTP mode Lost Boot Image.
Connect the TS directly to the LAN port of a PC/Laptop.
Set the IP address of the LAN card to 192.168.11.1 subnet: 255.255.255.0
you gave leave the gateway and DNS blank.
Download the TFTP image from ftp://24.153.165.234/array1/share/nas-recovery/TFTP%20Boot%20Recovery%20TS-HTGL-R5%201.20.exe
Unzip the contents to the desktop.
Deactivate all anti virus and firewall software on the PC.
Run the TFTP Boot .exe file and after a few minutes you will see 2 sets of blocks passed on the DOS screen.
After another few minutes the Terastation should be visible in the Nas Navigator and reporting as being in "EM mode"
Once this is the case download the 1.33 firmware (not 1.35) from http://www.buffalotech.com/support/getfile/TS-HTGL_1.33_1.32.zip
Again, unzip the firmware to the desktop and make sure you can ping the Terastation.
After 10 - 15 minutes the unit should be up and running, at this stage you should be able to configure the RAID for RAID5 (it is spanning as default)