Any backup disk that's connected to the computer is vulnerable to the computer suddenly becoming an untrusted actor. This could happen because the user types something dumb, a poorly-programmed application has a bug, the user falls prey to ransomware, etc.
One way to guard against this is of course keep the drive disconnected and only connect it briefly for backups. But this is inconvenient. It occurs to me that a better method would be an append-only drive. Your computer can write new data to it at any time, but is incapable of deleting or overwriting any past data, enforced by the drive itself. (Perhaps with some external override like a physical button on the drive that the user can press to allow deleting.)
Does anything like this exist? Of course you can simulate it with cloud storage, just program the remote server to only accept new data and have no API command to delete the old. But I'm asking about a physical drive that implements this natively.
Edit: Ah, I see there's a name for this, WORM drives. So my question then is, are there any of these made with modern technology? Capable of connecting via USB, storing multiple TB at reasonable r/W speeds, etc.