This article provides solution to an issue where upgrade fails due to error message "ERROR: Could not format alternate root"

While performing upgrade of a device, the upgrade might sometimes fail with below error message-

firmware writing failed. reason: Formatting alternate root (/dev/da0s1a)...
newfs: /dev/da0s1a: failed to open disk for writing
ERROR: Could not format alternate root

The below error is seen on running system snapshot media command-

user@router> show system snapshot media internal | no-more 
error: cannot mount /dev/da0s1a

This issue occurs when alternate partition gets busy and is unable to mount. To fix the issue, please follow the below steps-
user@router> start shell user root
root@router% umount /dev/da0s1a
root@router% exit
user@router> request system snapshot slice alternate

To verify if the alternate partition is mount, run the below command- 
user@router> show system snapshot media internal 

If the above provided steps does not solve the issue, please reboot the device to fix the issue.