There's always optical media or SD cards with physical write-protect toggles. (If you leave the optical media in the writer, though, conceivably an absurdly specific virus could cause the device to burn out existing contents.)
Needs to be accessible real time.
The drive controller could strongly enforce the onceness of writes. Possibly by attempting a read before every write. Or if that's not enough, with independently articulated read and write heads, the write head's movement could be mechanically restricted...
The read/write switch on SD cards is 100% up to the reader device to honor or ignore (seriously)