Comment 1 for bug 1431178

GPT fdisk relies on OS calls to determine the sector size, which is vitally important for interpreting GPT data structures. It appears that the NTFS-3g driver that Ubuntu uses is not returning that data, which means that sgdisk can only make a guess that the sector size is 512. (In fact, when dealing with disk image files, it would probably be better to provide a manual override for the sector size. I'll add that to the GPT fdisk "wish list.")