> Am I being silly, or could a hard drive full of random data be confused with a hard drive full of encrypted data by some member of the security services in the future? Would zeros not be a better bet?

I was under the impression zeroing, well, didn't fully zero—specialized tools can recover the former bits. If you write a truly random stream to the drive, it's much harder to recover the former bit versus the new, random one. To my understanding it's the same reason why XORing one-time-pads are utterly effective as a form of encryption (and you can throw away the pad).