If ur dvd unit is an Hitachi GD-7500, plus I am lead to belive one or two other models, bought as a non retail pack (Bare Drive) such as from a computer show or the like, their is NO firmware installed.
I bought mine as a bare drive and had to find the firmware on the net.
Once it was installed (use the latest version) it has worked great ever since.
I was informed where I bought the drive of this prob, hence the price I paid.
Best way to find the program is just type into a search engine "Hitachi GD-7500 Firmware" (without the quotes).
Took me about 10 min to find the latest version.