Technically true but misleading. It's true in the same sense as "if I plant evidence to frame you for murder, you could be found guilty of murder". I mean, sure, but that skips over the bit where the prosecution has to prove to a jury, beyond reasonable doubt, that you did actually commit the murder. Or in this case, that the hard drive is filled with encrypted data and that you have the key to it.
In particular, the implication that if the police find some random data that they think is encrypted, you can be convicted just on the assumption that it's encrypted and that you have the key to it (unless you can prove otherwise) is false. If there's enough evidence to raise a question about whether you have the key to something that could be encrypted data, the burden of proof is on the prosecution to prove you do have the key to it (and therefore that it is encrypted data) beyond reasonable doubt: s.53(3) http://www.legislation.gov.uk/ukpga/2000/23/section/53
(to be clear I definitely do agree it's a bad law, just not for burden-of-proof-reversal reasons)