I expect because you were downloaded such large torrents that it encounters a specific region on the disk with bad blocks, causing all this. The faulty part might have been there some time.

It is good that you had warnings and time to access it and act.

It is possible with hdd tools to scan through a disk and mark bad blocks as off limits, so the disk can be partitioned and formatted ok again. Personally though I would remain worried that more of the disk would fail later.

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