Is it FAT or NTFS. If it is NTFS, you will not be able to write to the drive, only read from it (http://www.linux-ntfs.org/)
. I have heard that there is some commercial software which may allow you to do this, but haven't tried any myself.
If it is a FAT partition, you should check your fstab for the umask option, which determines access permissions for the drive. It is basically the opposite of the chmod file permissions system with an additional 0 at the beginning, so umask 0777 means nobody can do anything and umask 0000 means everybody can do everything.