Why does dd change the filesystem on the external disk?

Question

I took out a 250 GB SATA disk from my netbook, filled it with zeroes with dd and connected it to my laptop via the usb adapter. It's correctly detected by fdisk:

Disk /dev/sdb: 250.1 GB, 250059350016 bytes
255 heads, 63 sectors/track, 30401 cylinders, total 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x475fa272

   Device Boot      Start         End      Blocks   Id  System

I created a primary partition and used the following command to create a ext4 filesystem:

mkfs.ext4 /dev/sdb1

Now fidsk shows:

Disk /dev/sdb: 250.1 GB, 250059350016 bytes
81 heads, 63 sectors/track, 95707 cylinders, total 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x475fa272

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048   488397167   244197560   83  Linux

Ok, no surprise so far. Next, I wanted to extract Ubuntu ISO image with the following command:

dd if=~/ubuntu-12.10-desktop-i386.iso of=/dev/sdb

To my surprise, filesystem changed to 'Hidden HPFS/NTFS':

Disk /dev/sdb: 250.1 GB, 250059350016 bytes
19 heads, 24 sectors/track, 1071046 cylinders, total 488397168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x475fa272

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          64     1542743      771340   17  Hidden HPFS/NTFS

What's the CORRECT way to extract ISO image to the external drive using dd with preserving the current filesystem?

Answer 1

You write to the disk (/dev/sdb), not to the file system you created (/dev/sdb1).

Since an ISO image already contains a file system (which you don't want) you can simply mount the ISO (with option -o loop) and copy the data to /dev/sdb1. That way the file system is preserved.

Answer 2

That's not what dd is for and that's not how dd works. If you want the contents of the ISO then you can loopback mount it and copy the individual files.

However, you seem to be wanting to copy the contents of the Ubuntu installer onto a hard disk that presumably you'll want to boot from. Again, that's not how it works.

Optical disks (CD/DVD) use the ISO 9660 filesystem with Rock Ridge extensions for UNIX and El Torito for booting.

A fixed disk (HDD/SSD/FDD) does not work with ISO 9660 filesystems and cannot boot from an El Torito image. You can't just copy the Live CD contents and expect it to work. You actually need to install Ubuntu.

If the computer you want to install to doesn't have an optical drive then use the USB installer.

Answer 3

I assume your laptop has a CDROM drive. If so, could you install your netbook's drive in your laptop (as opposed to connecting it via USB) and then install Ubuntu on it. After that put it back in your netbook.

Not sure how dependent an Ubuntu installtion is on the actual hardware so you might need to do something after the drive is back in the netbook to tweak things to work with the netbook instead of the laptop.