To burn file image to SD card using Ubuntu, first format card in Nook. That will erase everything from the card and set it up for use on your device.
Insert SD card into your computer.
Then run this command in Terminal: sudo fdisk -l
That brings up a list of your drives. By looking at the size of your drives you can easily see from the size which your SD card is. My SD card in terminal is named /dev/mmcblk0
First a warning, by selecting the wrong device it is easy to overwrite your hard drive by mistake, make sure you know which drive is which.
Then to write the image to your card you would use: sudo dd if=~/filename.img of=/dev/mmcblk0
This is assuming the image is called filename.img and it is in your home folder, adjust your path and filename to suit.
For example, my full command was: sudo dd if=~/NookHD-bootable-CWM-6025-for-emmc-stock-4GB-rev2.img of=/dev/mmcblk0
It will ask for password and then not really do anything. Do not worry, it is writing. However, It could take a while depending on the speed of your card reader.
When it is finished, unmount and remove SD card, and then reinsert. The card will have been renamed to boot.
From this point on just follow the instructions in
this thread
Enjoy!