This case is interesting. If I'm reading https://en.m.wikipedia.org/wiki/United_States_v._Hubbell correctly, the fifth amendment only applies if "they don't know what they're looking for." In this case, because there is (enough) evidence of CP on his computer, they are subpoenaing him to produce the unencrypted drives. In some sense, they're not asking for a password - they're asking for the drive contents, which the…
I don't see how this is different than having circumstantial evidence that someone is a murderer, so ordering them to lead you to where they buried the body.