Why is the error, dd: /dev/rdisk1bs=1m: Operation not supported, popping up while trying to instal ubuntu on usb?

I am trying to install ubuntu onto my flash drive using the instructions from this website: http://www.ubuntu.com/download/help/create-a-usb-stick-on-mac-osx.

After step 8, the terminal asks for my password, which it accepts and then pops up with this error message:

dd: /dev/rdisk1bs=1m: Operation not supported.

I have also tried making the last m in that statement capital and then the system does not ask me for my password but the error message still pops up. What is happening and why?

If you clicked the little eject icon in the Finder, this will unmount and eject the usb drive, which will give the error you saw. You want to umount only.

Unmounting can be done in Apple's Disk Utility using the Unmount button (in toolbar next to Eject), or it can be done as Marcel answers below from the commandline.

sudo diskutil unmountDisk /dev/diskN

If the above does not help (as it did for me), try

sudo diskutil umountDisk /dev/diskN

That did the trick for me.

If the disk is still mounted, you'll get a 'resource busy' error. If it's been ejected, you'll get the 'Operation not supported' error. You need to go into disk utility and UNMOUNT the partitions on the drive but do not EJECT the destination drive. Hope this helps as it can get kinda frustrating and the errors aren't always particularly helpful.