Samsung "Magician" software...
Now you see it (your data) and now you don't.... (Poor name for a product, IMHO)
Sorry, I feel for anyone that has been caught in Sammy's FUp.
Coincidently, I have a brand new 850 Pro waiting to install into my laptop as an upgrade in speed and size (scheduled for this weekend). Wow, I am fortunate to have waited a couple of days before doing so. Four or 5 years ago I got caught in the Seagate firmware bollox (something to do with the firmware's log files at powerup IIRC???) with their 1TB drives. I have 4 of them, but again, fortunately I powered them all off before they became bricked, waited for a firmware fix and was able to apply it to all of them. The drives are still working to this day.
Now I wait to hear from lord Sammy for a fix. Samsung is slowly creeping up on my list of shit companies not to do business with.