BIOS flash on a gtX260

So recently i got a gtx 260, that happens to be a zotac amp!2 overclocked card. Now, i'd like to overvolt it to 1.18v or 1.20v
After a really hard time trying to get a bootable usb drive, i managed it and loaded nvflash and the bios rom file. So i restart the computer, open the boot menu, select the usb flash drive and write nvflash -4 -5 -6 gt200.rom (gt200.rom is the name of the bios file), i get an error saying: ERROR: EEPROM not supported (something like this). Now i don't know what to do...   can someone help me? If needed, i'll post the system specs