There's a burden of proof here the court needs to meet before holding the defendant in contempt.
For starters, it's reasonable to assume the defendant owns the hard drives in question if they're in their possession, irregardless of their testimony otherwise.
Given that piece of information:
1: The court has to prove the disks are actually encrypted. It is not merely enough for the cops to pick up the disks, see some garbled data, and determine it's encrypted. Now if you're using a file level encryption protocol that leaves enough un-encrypted stuff on disk that you can identify the filesystem and the file encryption, then you've met the requirement. If you are using full disk encryption, especially something designed to hide the data and filesystem from anything but a forensics package and even a forensics package see's garble, then there's effectively no way to tell the disk is actually encrypted or with what.
AND
2: They have to prove the defendant, at some point, had the encryption key. That requires proving the method of encryption and key generation. With a door lock, you know there's a key. With a safe combo, you know the combo could be 12 digits and broken up between a dozen people. With an encryption system, any combination of things you know, are, or have could be part of the key. Compelling the defendant to reveal all of that is absolutely a violation of their 5th amendment rights.
Example:
Lets assume we're using windows EFS. Lets further assume analysis of EFS indicates the user named "YOU" owns the account. Furthermore, lets assume there are files that have date modified dates within the end users folder that indicate they had logged in the day prior to the search warrant being served.
You give the court the "I don't remember" line.
In that case, forgetting the password is destruction of evidence, not contempt. If a key escrow is used and they can prove it, same deal, destruction of evidence.