You're creating a dichotomy that doesn't exist. What actually happens is that there is an intent element to crimes associated with destroying evidence. So you might get in trouble for purposefully destroying evidence, or in some cases negligently destroying evidence (e.g. a company that didn't have a proper data-retention policy). You usually can't get in trouble for accidentally destroying evidence. Then, you testify as to your intent, and the jury gets to decide whether you're telling the truth, making inferences from your circumstances. People might believe you forgot the password to some drive you never use, but probably won't believe you forgot the password to the drive holding the bank codes for all the money you embezzled.
And what if the police just think the drive holds the bank codes for all the money I embezzled, but is actually an archive of amusing cat pictures I forgot the password to a year ago?
Seems to me that the only way to reasonably apply "innocent until proven guilty" there is to only convict if they know what the encrypted contents are, and if they can already prove that beyond a reasonable doubt, why do they even need you to decrypt them for you? Conversely, if they don't know the contents, then they may well be innocent, and the password innocently forgotten.
