1Tb drive changed data - help!

arielzuk01

Prominent
Sep 30, 2017
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I use a pretty regular setup:
1 SSD 120Gb drive for my Windows 10 OS, 1 regular 200Gb hard drive as backup to the SSD, and a single 1Tb hard drive to store everything else.
I've bought my new computer back in mid or end of 2016 I believe, and I formatted a 1Tb drive from my older computer to use on the new one. I wanted a clean start, no more clutter.

Yesterday it seems that my PC did a complete make-over entirely on its own. My user switched to that I used in my old PC, and despite it saving my legit user in its "Users" folder, I have no access to my actual user. The current user it uses has data from at least early 2015, and its files seem to match some of those used on my Laptop, which I only ever connected to my PC to transfer Windows 10 installation files.

My 1Tb drive also switched its data to that from the user from 2015. All my games, programs, and whatnot have been erased, and exist only those that I had back then.

How do I get my PC to recognize my legit user? And how do I restore my new PC user's data on the 1Tb drive?

EDIT: Forgot to mention that my computer got considerably slower. It takes 1:30 minutes to boot, but before that it took roughly 15 seconds.