Hi Yogesh, first of all when you say mounted the drive does it mean pen drive? Is it formatted with fat or vfat for that matter? In any case the fat or vfat file systems do not have any file permissions on them. If this is not the case and you are using one of the linux filesystems like ext4 or simillar then use 'lsattr filename.txt' if anything shows up (anything with 'i' or a simillar alphabet then you can use 'chattr filename.txt' to change the attributes.
Since we do not know what is happening I have given you some pointers. Hope this helps.
And welcome to LQ