Why on earth are you not using SSD's? The HN footprint can't be that large. The extra speed and reliability from a pair of SSD's has to far outweigh the costs.
I'd guesstimate that the READ load is served practically entirely from RAM (file cache) and the WRITE load is non-critical enough that it's done "eventually consistent" (e.g. synchronous_commit=off in PostgreSQL, or fsync=off elsewhere) - or at least that's how I'd run it. YMMV.
