1. Can I backup my whole C drive (including OS) so I can restore to a bare disk after a crash?
We cannot add system folders for Backup. Even if it allows you to add the system folders through Add or exclude files and folders UI, system folders/files will not be listed in the Preview UI and will not be backed up as well
2. If I already have a backup file (some data) either at Online Storage or external or secondary hard drive, how do I restore from there?
To Restore the data from Online Storage : Navigate to Backup Pillar in the Main UI ->Restore Files->Choose the Backup set from the Restore From category (It will display the Online backup sets as well) ->Choose the File Type and then do a Restore from there
[Internet connection required for Online Storage Restore alone]
Incase you want to do a Restore from external drive : Connect the External drive and then follow the above steps
3. If I have to reinstall OS first, can I download trial version and use my activation key to make Norton 360 fully working?
Yes you can use your activation key to activate the trial product. Make sure you connect it to the same Norton account to make Norton 360 back up fully working