This can occur when you do not have enough space on the C:\ drive. Even though the ISO might be created to a external or shared network device, the boot wizard still creates files temporarily to the C:\ drive.
Remove files to clear up space on the C:\ drive, and try to create the ISO again.
NOTE: Symantec does not provide support for modifying the registry. Please modify at your own risk.
First, back up your registry. Open the registry, and go to File > Export... and save the registry to a safe location.
To solve the issue, you can direct all temporary files to another location (such as a mapped drive, or external USB device).
Browse to the key:
And modify the key:
TEMP (the default location it is set to is %SystemRoom%\Temp)
Modify this key to point to your external or shared...