How did you managed to do that? Which firmware have you flashed and using what software tools? Normally you shouldn't be able to do that sort of thing ... either the drive or the flashing tool will reject it!
Is the drive still recognised by Windows?
Its possible, but what exactly are you trying to do with it (just to make sure you don't make things worse than they are)?
You can use DevilsClaw's 'flasher' to extract the main and the core parts of the GH20NS15 firmware: http://sourceforge.net/projects/flasher/files/.
Normally its the main part which you would need to flash back.
Put the firmware in the same location with flasher and run the tool in the command line as:
flasher -r firmware.exe