Sorry, but legally, the latter should be the only standard of truth. If he exposes himself to a higher standard of guilt, then he is incriminating himself.
To my mind private spaces (be that my house or my hard drive) should have some protection, but it seems reasonable that that is less than my personal freedom.
I have no issue with a warrant being issued on a balance of probabilities basis in order to find evidence to convict a person based on beyond a reasonable doubt.
And all this ignores the possibility of discovering further crimes and accomplices by investigating the contents of this drive - if there is a balance of probabilities likelihood of find those on the drive I don't see any problems with compelling this to be revealed.
The problem with this is it isn't consistent with how the law works in other cases. For example, A judgment of 'guilty' is considered absolute, not probabilistic.
It's a foregone conclusion technically that the illegal content is on the hard drive. His guilt is not a foregone conclusion (not in the US anyway).
If you visit https://www.justice.org/sections/newsletters/articles/fifth-... and search for "foregone conclusion" you will get some good info.
The file hashes basically takeaway any good self-incrimination argument he could make and there might also be evidence of further criminality on the hard drive.