The forgone conclusion is that evidence likely exists on the drives and as such the judge issued the Writ pursuant to the All Writs Act. But to prosecute someone in a court of law it takes evidence, admissible evidence, and conclusions that it likely sits on an external hard drive is not in and of itself evidence of pornographic images of children.

Doesn't this conflict with the precedent set by, for example, safe combinations?

If I remember correctly it does but I cannot recall what the courts reasoning For deviating from the safe combination decision.