Hitachi just shaved 2.5mm off of the top of its 2.5-inch laptop hard drives, producing a line of 5400- and 7200-rpm, single-platter HDDs called the Z series (Z is the height axis, get it?) with capacities of 160 to 320 Gbytes. (Press release here.) Although there are standard specs for a 2.5-inch drive’s width and depth, height is not part of the size spec but some heights have become de facto standards. Initially, 2.5-inch HDDs were taller but the 9.5mm height became popular as notebooks got slimmer and slimmer. Now 2.5-inch, 12.5mm-thick HDDs with multiple platters are only used when drive capacity is more important than slimness. Hitachi has decided to terminate all 9.5mm-high, 2.5-inch HDDs in favor of drives with the 7mm height for single-platter HDDs. Hitachi will still offer 2.5-inch, 9.5mm drives with multiple platters.

The 7mm-high drives are supposedly plug and mounting-screw compatible with the 9.5mm-high HDDs, which allows Hitachi to replace all of its single-platter, 2.5-inch HDDs with 7mm units. There’s also supposedly no cost adder for the smaller drives, and no discount for the reduced amount of material used to fabricate the drive. Going forward, the single-platter drives will just be physically smaller.

If Hitachi’s new “thinner” 2.5-inch HDD form factor becomes popular—and in a world where you can never be too thin or too rich, the smaller form factor has a very good chance of becoming popular—then there are clear implications for SSDs that try to emulate the form factor of existing HDDs. If laptop, netbook, and even iPad cloners decide they like Hitachi’s new 7mm height, which is likely, then SSD vendors will need to emulate that 7mm height because new drive slots will only accommodate HDDs that are 7mm high. To paraphrase an old saying, you can’t shove a 9.5mm drive into a 7mm slot.