My Nextbox 3.0 (Cisco 9865) HDD failed a couple weeks ago and I'm happy to report that I've been able to teardown the unit and replace the drive without issue.
I'll break down my steps should anyone else wish / need to replace the drive as I did. The overall time from start to finish was around 60-75 minutes.
You'll need a Torx 10 and Torx 15 sized security screwdriver. I used the Husky multibit screwdriver set from Home Depot (~$16).
Plastic spudger or other soft flat pry tool to get the exterior case off.
Any off the shelf 1TB SATA drive. I purchased the Western Digital Blue for $73.00.
-The exterior of the unit has two Torx 10 sized screws on the left and right side, remove these
-Flip unit upside down where you will see two black plastic latches holding the lid in place. Use the plastic spudger to lift both and then slide the top case off.
-You'll now have the main unit exposed. You'll see the main electronics are covered by a safety plate (something the 8642HD does not have). There are now 8 additional screws which will need removal. Six on the top plate and two on either side.
-Once the screws are removed, use the spudger tool to lightly spry the metal clasps holding that plate in place. Lift it off gently.
-The electronics to the whole unit will now be accessible.
-Use the Torx 10 screw bit to unscrew the four golden screws holding the hard drive plate in place.
-Gently remove the main power/sata coupler from the drive before lifting out
-You'll now have the hard drive removed from the unit, but still attached to the support cage
-Use the Torx 15 to remove the last four screws and remove the drive.
Now follow the steps in reverse installing your new drive. As Cisco uses a proprietary hard drive formatting, you do not need to preformat the drive.
Once you've got everything back together you'll need to reconnect it to power, HDMI and cable in. This part takes around 20 minutes so you'll need to be patient.
The unit will boot back up and see a new drive. On the LCD screen HDDF will be lit up -- meaning the unit will format the drive. You'll then see cd## as it formats and downloads to update the unit. My unit rebooted about 3 times throughout the process and then eventually displayed the current time. After turning the unit back on, I was informed it was no longer authorized and had to call Rogers to re-authorize the unit. I'm guessing the HDD may store some auth data. Maybe it was just my unit - either way, it'll be your longest step if necessary...waiting on hold.
Good Luck and have fun!