The unclear part seems to be how strong the evidence needs to be that the device is yours and that the evidence is on there. In this case, his sister testified to both. But would it be strong enough with forensic evidence alone? Unclear.

> As far as I understand things you've lost the case by that point anyway.

Perhaps, but there may still be significance to what's on that drive. There may be incriminating evidence there for other crimes for which you're not yet being prosecuted.

> Even then, I believe there would still be the additional issue of demonstrating that the defendant actually knows the password.

They're saying the sister's testimony was sufficient to prove that he knew the passwords previously.

Proving present-day capability to decrypt doesn't seem to be necessary, at least in the article I linked.

> The federal court denied the Motion to Quash and directed Doe to unlock the devices for the investigators. Doe did not appeal, but he refused to unlock some of the devices, claiming that he had forgotten the passwords. He was eventually held in contempt by the District Court, and the Court ordered that he remain in federal custody until he was willing to unlock the devices.

The accused claimed he could not decrypt the hard drive because he had forgotten the passwords, but he was still being held in contempt.