That is a neat idea. Hard drives could do dedupe from the ECC they calculate for each sector. The main issue with that is that the current ECC is optimal for detecting bit errors but doesn't have the same kind of statistical guarantee of uniqueness that SHA256 or MetroHash has. You need to be VERY confident of the statistical properties of the hash used in dedupe if you are going to increment the ref count of the block hash instead of writing the data to disk.