handling image on ntfs: "Error 38 when determining sector size! Setting sector size to 512"

[Aapo Rantalainen]

Creating two similar images, with empty GPT. First image in ext-filesystem, second in NTFS.

/tmp = ext4
/tmp2 = ntfs

dd if=/dev/zero bs=1M count=10 of=/tmp/test_ext.dd
sgdisk -og /tmp/test_ext.dd -U EBD0A0A2-B9E5-4433-87C0-68B6B72699C7

dd if=/dev/zero bs=1M count=10 of=/tmp2/test_ntfs.dd
sgdisk -og /tmp/test_ntfs.dd -U EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
#This gives 29 times: "Error 38 when determining sector size! Setting sector size to 512"

md5sum /tmp/test_ext.dd /tmp2/test_ntfs.dd
f90b33efcce9385f6bc2d13d9cb62365 /tmp/test_ext.dd
f90b33efcce9385f6bc2d13d9cb62365 /tmp2/test_ntfs.dd

gdisk -l /tmp2/test_ntfs.dd

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /tmp2/test_ntfs.dd: 20480 sectors, 10.0 MiB
Logical sector size: 512 bytes
Disk identifier (GUID): EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 20446
Partitions will be aligned on 2048-sector boundaries
Total free space is 20413 sectors (10.0 MiB)

Shows 19 times: "Error 38 when determining sector size! Setting sector size to 512"

[Rod Smith (rodsmith)]

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.")