@TVeblen, at this point I need a clarification to clear my own doubts since I am not sure about it.
Can we not connect an external optical drive and check whether it works without any problems? If it does work properly can we not categorically say yes there is no problem with the OS and the internal drive is dead. Or is it not?

.....The only problem is that each piece of hardware in a PC has a unique identifying number in it's firmware and a corresponding set of key entries in the registry. So if there were a corruption in the keys for the internal DVD drive, that corruption would not (should not) pass along to the external DVD drive, which would create and have it's own set of registry keys.
......