Well, some powerful legal scholars and the court cases they describe disagree and state that in this exact predicament
the 5th does apply. The so called Fischer test fails on point 3 and the burden is on the government. They warn that if this were not the case one could be jailed for contempt, and that is exactly what happened in this case. Likely the incarcerated lawyer did not push this hard enough, or the judge chose to ignore it. But without all the details we may fall short of the understanding needed to make an argument one way or the other. But understanding this person may not know, may have forgotten, or some other individual may have applied the FileVault passphrase, or may just be such a willful person (and two years makes that self evident) that he will never reveal the passphrase if he knows it; he should be released. The current zeal the courts are using to make encryption a worthless technology is unwarranted. Least people think this is a new problem, read the paper from the above link. Thomas Jefferson invented an encryption for mailed messages that was not broken for 100 years apparently. this problem is not modern at all and has been tested in courts since the beginnings of the U.S.A.
Consider if this hard drive contains emails between this defendant and his lawyer, thus privileged communications. Or contains material related to confidential informants. Disclosure in open court could be disastrous
... and in the former would violate the defendants rights in yet another way.
We have lost sight of the American concept that it is better to let a dozen guilty people go free that to jail one innocent man. Our prisons have uncounted numbers of innocents, some lucky enough to have people interested in freeing them to preserver until they are vindicated. Some innocents die having exhausted all appeals and they are executed. It is very sobering to look at The Innocence Project
and understand many of these people lost decades behind bars while innocent.