Secondary hash function utility, regarding data destruction? Given that hash functions are inherently irreversible, would they not then be useful in applications where it's desired to overwrite sensitive data? (i.e. Darik’s Boot and Nuke) 

Specifically, why should hash functions not be used for data clearing?
Specifically, why should hash functions not be used for data clearing?


I take it you mean that we would overwrite the storage with output produced by feeding its sensitive contents to a hash function.  The problem then is that this output, being a function of the data you're overwriting, therefore provides information about that input.  

In particular, it could be used to efficiently test hypotheses about the content of the overwritten data.  This is the same principle as password cracking; just like password hashes allow you to quickly test password guesses, the overwritten disk sectors' contents could perhaps be used to test whether the disk contained or not some specific data before being overwritten.

You may think that this sounds like a very unlikely attack, and you could well be right about that.  But why allow it in the first place?  Particularly when the alternative is simple enough: overwrite with random or pseudorandom data that has no correlation at all to the sensitive contents you're overwriting.