Two nightmares from when I was a child:
- Earth is suddenly in orbit around Jupiter and it’s filling the sky, everything is bathed in Jupoter-shine light.
- Earth is visited by human-sized black amorphous blobs that just appear and disappear at random, destroy anything they intersect with, instill abject terror in anyone who sees them.
If you’re using LVM, ZFS, or Btrfs then you can use their features and tooling to migrate data from one disk to the other, assuming you’re able to connect both at the same time.
I’ve done this online with btrfs several times now and it’s quite painless, admittedly only for self hosted stuff.