In series, definitely.
The pressure drop of an HDD block *should* be minimal, and wouldn't have a significant impact on coolant temp.
Otherwise, the heatload from an HDD is quite minimal (at least the heat transfered to the block) and so it's really not going to matter a whole lot how it's set up, but putting it in parallel, would cause a significant drop in flow for the rest of the system, so "in series" is always advisable.
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