USBs exist that have physical write switches, and there's also old-school media like CD-Rs which can in theory be used too. Hell, with a bit of fiddling, you c

USBs exist that have physical write switches, and there's also old-school media like CD-Rs which can in theory be used too. Hell, with a bit of fiddling, you could probably design a one-way RS232 serial system by simply not connecting the TX pin on a downstream device. Then for good measure snap off the pin and block the corresponding socket's hole with superglue so the cable cannot be reversed. There are also write protector devices which mirror a block storage device as read only, but will not issue a write, as used for forensics.