If virtualbox was successful using the ISO file, then very probably you are experiencing a burning problem on the DVD media ...
AS
Yes you are right. I thought my drive is OK since it is loading downloaded CD. I tried my older remaster dvds which were working. My drive failed to detect many of them. I tried to test the recent disc, which I was thinking not working in my friend's computer. There it was working without any problem.
UNetBootIn actually mislead me. I thought that I can use unetbootin to boot my remasters. But it is not working even with my older remasters.
I mark this as solved, since it is my drive problem.