windows 10 (October 2018 update) (version 1809) deleted all my files

I lost lot of files with windows 10.I have Mac now, no problem. Windows 7, number 1

To avoid the problem of issues like Microsoft wiping out my data or a piece of ransomware encrypting it, my important data is all on a home server run by Ubuntu Linux.  The Ubuntu server backs nightly to an external hard drive using Back-In-Time providing an incremental backup.  The external drive is in NTFS format, so if a file needs to be recovered, it can be recovered either by the server or any Windows computer that reads NTFS partitions.  A file can be restored from any date that has been retained.  The retention policy I setup at home is keep all snapshots for last 2 days, keep one snapshot per day for last 20 days, one snapshot per week for last 50 weeks, one snapshot per month for last 24 months and one snapshot per year for all years. 

I pull a drive from service after 2 years and keep in a safe place and put a new drive in place.  I also backup once a month using Back-In-Time on my primary Ubuntu workstation. 

The really big advantage of this approach is in cases of a ransomware attack on a Windows system on the network, Windows cannot connect to the backups on the workstation and unlikely to be able to connect to the backups on the server. 

I do not store important data on a Windows computer, other than copy large files from the server to the computer to manipulate and then upload back to the server.  My Windows installations are primarily operating system and programs and most of my data is on my server.  I do the same with my Ubuntu workstation and test systems.

I think a mod should review this thread, sticky the reference to the RCA and the reply that includes steps on using a recovery utility, and lock this thread.

It's attracting low quality responses, becoming an echo chamber, and giving the false perception that a significant portion of users were affected by this (the actual stat is between 0.5% and 0.05% of all recipients of the bugged update, i believe)

MS: bad on you for not detecting a 0.5% edge case (sarcasm)

MS: good on you for stopping the update when you did, fixing it, and then re-releasing

End User: you have every right to confidence that your user data is precious and that a vendor update should not affect it

End user: bad on you for not backing up your super-important data

Good news is that the problem is understood, and resolved, and that there are free tools to attempt to recover your data. There are also paid services out there if you dont feel comfortable with doing this yourself. You may also have a case for sticking MS with the bill. But thats between you and MS.

800 GB of data that was obviously important to you ... I'm sure it was backed up.

"I think Microsoft should have to give everyone free copies of Windows for life and assume responsibility for their screw up."

In the not-too-distant future Windows will be free. It has little time remaining as a standalone product. Windows is moving to the cloud where it will be a service, like ChromeOS is now.

Much has been written here (and elsewhere) about the steps you need to take to recover your files, if they are recoverable. Microsoft did, in fact, step up.