Question about HD plates data density

I’m planning to replace the 2.5" WD 320GB HD in my laptop (actually I would not need it, but the drive did more than 400k head load cycles in less than 2 years because of some **bleep** ACPI setting and I don’t trust it anymore, so I prefer to match it with a twin that I have at hand and make a RAID1 system in a dedicated external box) and I’m thinking about something bigger than 500GB, but I’d like it to have the *least* data density because of the reliability point of view.

I know that WD managed to put 500GB in one plate (with two r/w heads employed), so I’m wondering whether their 2.5" 750GB HDs have two plates with 3 r/w heads (so 3 sides used at their max data density of 250GB per side), or the 750GB are distributed uniformly among *all* the 4 sides with a lower data density (which would be my preferred option).

Many thanks in advance.

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