Where did all my partitions go!?

Mar 9, 2018
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So recently I upgraded my PC, well not I another person did it for me (dad didn't let me do it). I didn't buy any HDD as I already had one 1tb. Now when we were installing win 10 on my old, formatted HDD, he did something, I don't know what, but when my computer booted up I saw only the root partition (local disc C) 200gigs. The rest 800 gigs of partition just disappeared. I know it's not a HDD problem. How can I get that 800 gigs back? Without formatting of possible, if not then fine.