1) external magnet messed up heads movement closed control loop (internal drive's magnets + voice coil + positioning marks on the platters), and as a result head actual position became different from what controller thought it was.
2) control loop tried to correct itself, repeatedly failing, all while head kept moving across the platters, while controller was still thinking it's elsewhere.
3) controller was writing to disk, so it overwrote data in highly irregular tracks that followed erratic head dance. some of that overwrote positioning markers.
4) when recovering, even though 99.9% of data was still there, controller failed to position the head on erased markers, so reported "unreadable sectors"
1) external magnet messed up heads movement closed control loop (internal drive's magnets + voice coil + positioning marks on the platters), and as a result head actual position became different from what controller thought it was.
2) control loop tried to correct itself, repeatedly failing, all while head kept moving across the platters, while controller was still thinking it's elsewhere.
3) controller was writing to disk, so it overwrote data in highly irregular tracks that followed erratic head dance. some of that overwrote positioning markers.
4) when recovering, even though 99.9% of data was still there, controller failed to position the head on erased markers, so reported "unreadable sectors"