actually in the previous post , i want to explain the reason.
H drive is not the cd rom and any read only type disc. it is a Hard disk.
this partition is in the second hard disc i have.
at first, i have tried to create files on H drive.
when it is throwing exception, i tried on D drive.
in those trails, i forget that i am using a drive (D:\\) and not a file (D:\\sample.txt) etc.
that is the whole story.