Dammit, that's hard. A lot of words to lose.
I once lost my entire wip when I was trying to do some icon-shuffling on the desktop. I dragged the icon, dropped it and... poof. Gone. Nowhere to be found. Entire wip. BUT I was using Scrivener with auto-backup on After 5 minutes of blind panic, five minutes of Googling, five minutes of searching the hard drive for the backup files, and five minutes to restore, I was back in business. Didn't lose so much as a full stop. But I never used that laptop again lol. Never trusted it.
What I do now is... use Scrivener with auto-save on. At the end of each day, I copy to Dropbox {*} and a thumb drive, which I keep on my keyring. I have two thumb drives, in case one fails, and alternate using them. When a wip is finished, it gets copied to the main computer, where it falls under the automated backup process to external hard drive and the cloud.
{*} It always makes me shiver when people say they keep their wip in Dropbox. Dropbox is a wonderful invention, but it's a synchronisation app, not a backup app. If you accidentally delete your wip on your computer, Dropbox will helpfully delete it everywhere, and then you just have to hope you've got the kind of Dropbox that keeps old versions. Use it for backup, or be rigorous about keeping multiple backups elsewhere, but never, ever trust it with your only copy of a work.