If you don’t want to void Nook Color warranty, You can run CM7 from SD card. When you want to use original Nook OS, just takeout SD card and reboot Nook Color.
4. Micro SD card (preferably 2gb, 4gb and 8gb)
5. Nook Color
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How to create microSD bootdisk
For writing the resulting image to the microSD card, use “dd” in Linux and Mac.Win32DiskImager is a program that can be used in windows for writing the image.
Steps to Follow
1. Download clockwork Recovery file
2. Download and unzip win32DiskImage
3. Copy win32DiskImage.exe in the folder and Run it, the following screen will appear
5. Now click the disk drive letter label that corresponds to the microSD card.
6. Click Write.
7. Click Exit at the completion.
8. Eject the SD card safely.
9. Re-insert SD card.
WARNING: Be sure to create the back up before this process because the microSD card will be completely erased.
Creating Bootable CM7 SD Card
1. Copy CM7.zip which you downloaded earlier
2. Remove SD card from the computer safely.
3. Power off the Nook Color and insert SD card.
4. Power on the Nook Color and it will show you the progress of CM7 installation on SD card.
5. Now choose “install zip from microSD card.
6. Once Installation of CM7 on SD card is complete, Nook will power off.
7. Turn it back on and it should boot CM7.
8. Make sure during startup setting to correctly setup Wi-Fi.
In case of any problem, feature requests, updates, or help on this latest ROM development, you can contact the thread at XDA-Developers forums.