Despite Doe’s refusal, f orensic analysts discovered the password to decrypt the Mac Pro Computer , but could not de crypt the external hard drives. [...] The Forensic examination also disclosed that Doe had downloaded thousands of files known by their “hash ” values to be child pornography. 3 The files, however, were not on the Mac Pro, but i nstead had been stored on the encrypted external hard drives. Accordingly, the files themselves could not be accessed.

IMHO, if they really wanted to charge him and take him to trial, they have plenty of evidence. I doubt a jury would be sympathetic to the "I forgot my password" argument and the logic would be that if the files _were not_ on the external drive then he would willingly decrypt it to prove as much.