The resulting CM10 install will run entirely from the SD slot, without altering the contents of the Nook Color itself. The NC will always try to boot first from the SD slot. To load the stock, on-board operating system, hold n/home right after powering on and choose "emmc" and "normal" from the boot menu. For all intents and purposes, this arrangement preserves your warranty, because when the card is removed there is no indication that the NC was ever anything but 100% stock.*
I make no guarantees, however, regarding how this process will impact your warranty or your device. YOU are responsible for changes you make to YOUR device. /disclaimer
Some people refer to this arrangement as dual booting, though more advanced users can also install and dual boot both OSes directly from the NC.
- You will need a Micro SD card 1GB or larger. Class 4 Sandisk cards 8GB and larger have provided the most consistently stable installs. Class 2 and unclassed Sandisks have also performed well, but most other cards, while allowing you to install CM7, will have performance issues. More information.
- You may need a USB Micro SDHC card reader. All-in-one readers found on laptops and some PCs are often unreliable for writing a disk image. Some people have success using the NC, a phone, or a camera as a reader, but it's not recommended, and may render the card unreadable if the process is interrupted. More information.
- In Windows, you will need a utility such as WinImage or Win32DiskImager to write this image to your SD card. In Linux or Mac OSX use the dd command, being sure to write to the entire device and not a numbered partition. More detailed Mac instructions (thanks to justChris).
- Download the disk image (thanks to leapinlar for the CM9/10-compatible revision) in compressed .zip format. It's an approximately 8MB file which you will extract into an approximately 200MB disk image. More info on CM9/10 compatibility.
- You will also need a CyanogenMod 10 ROM from this list. This installer is made to work with CM 7.1 and up, not the older 7.0.x releases.
- Finally, if you wish to activate a Google account for the Android Market, GMail and etc., you will need the CM10.0 Google Apps package from this page.
- Write the extracted image file to the SD card. In WinImage, the command is Restore Virtual Hard Disk Image on Physical Drive under the Disk menu. Be sure to choose the file type All Files so you can see generic-sdcard.img. It may take a few minutes.
- Copy the CM10 update file (as is: do not unzip) to card.
- Safely remove/eject the card from your computer, place it in your powered-off Nook Color, and power-on. The NC will unpack the CM7 ROM to the SD card, informing you of its progress, and power off when it is done. CM7 is now installed.
- If you wish to install Google Apps remove the SD card and return it to your PC.
- On your PC, copy gapps-*-signed.zip file (as is:do not unzip!) to the card's boot partition (the only partition Windows can now access).
- Safely remove/eject the card, return it to the NC, and boot into CM7.
- Set up a Wi-Fi network (important!), then hold and release the power button to bring up the power menu.
- Choose Reboot > Recovery. The NC will now reboot and install Google Apps, which can happen very quickly! You should be prompted to set up a Google account when you boot back into CM7, but if not, check Settings > Accounts & Sync before assuming the package did not install.
To add books or other media to the card's storage partition, you must boot into CM7 and connect to your PC via USB, and turn on USB storage by tapping through the "USB Connected" notification on the NC. If you connect the card to Windows with a card reader after writing the image in this guide, Windows will tell you there is only 200ish MB free on the card. More info on working with bootable SDs.__________________________________________________________
*The one way you could inadvertently install something on the NC itself is to open ROM Manager and "Flash ClockworkMod Recovery," which would install CWM on the NC's recovery partition. Neither ROM Manager nor CWM are necessary or useful to maintain a CM10 SD install; leave ROM Manager alone or uninstall it until and unless you are ready to replace the stock OS with CM10 on eMMC (embedded Multi Media Card, shorthand for the NC's internal flash storage).