This has not stopped courts from accepting digital evidence and the creation of a "digital forensics" industry. Perhaps the only persons who could successfuly call this into question are the same people, so-called "experts", who are supporting its continued existence.
At least in Sweden (where I can follow the discussions easily) the defence are raising questions about the legality and to some extent, the validity of the evidence from EncroChat. So far it seems seems the courts are accepting the evidence.
Also, it is quite amusing to read the EncroChat logs from some of these trials. The user names selected and the messages sent sometimes shows very bad opsec.
That's interesting, where can I find those?
Imagine a scenario like FBI Bob and EFF Alice show up to image a suspect's hard drive. Both of them image it, Alice hashes it and throws away the image, Bob keeps the image, then if there's a dispute, Alice is called in and provides her hash and if Bob's image doesn't match it, the tampering alarms go off.
The hash verification you describe happens (without independent oversight but enugh to protect against a single bribed/malicious officer) and it does protect from post-factum altering of any digital evidence; you try to do the early (physical) parts quickly, the majority of analysis work comes after you have the images and their hashes - but everything from that analysis can reproduced from the verified images if it's disputed.
If course, if they already trust each other (including all the used tools), then the whole excercise is moot.
They would need to trust that the bridge is not malicious. And that is a whole other rabbit hole. But I think it is possible for them to attest the firmware/gateware running on it, through some convoluted cryptographic ceremony.
They'd get a live notification of tampering, and could have independent signal blocksers that could physically block the command from actually arriving at the drive (assuming they use a few hundred meters of coiled fiber as a delay line).
The receiver on the drive side could even be a single phtotodiode, which could be made to allow easy verification with, say, an electron microscope if you're really paranoid. There are probably ways to use field-suitable technology if you only need to ensure the photodiode has the same structure as what you expect.
Cryptography won't help you with trusting hardware. Delays and intervention-ability would help, though.
But in any case, this is not really the level of concern here. It's equipment that tampers with the device. The only way to be sure is to roll your own. Which holds for both sides. So the perfect systems needs to be created by two adverse parties, which means it's impossible to do. Qed.
(In the real world with physical proof this is different since tampering is much harder and it's a problem worked on for centuries. It's not bullet proof either but much more mature.)
Q: How could you verify that a write blocker was actually used, and that it functioned as described?