If you like the HD’s to compete with current SSD’s, at least for the short tem,
you may want to have a double, triple or quad actuator arm in a small 2.5 factor disk
We asked a very similar question of WDC many years ago,
and we even found patent drawings of this very thing.
WDC’s Customer Support replied promptly, and professionally,
to say that a dual-servo HDD would be much more expensive
to manufacture, and it would also be much more prone to fail
simply because of the larger number of moving parts.
And, believe it or not, the solution is really staring us in the face:
use 2 x HDDs in RAID 0 arrays, if you want 2 x actuator arms!
Along the same lines as WDC’s reply, the industry was also
exploring much success with short-stroked partitions
particularly after perpendicular magnetic recording became
the standard for new rotating platters.
Here is a very revealing graph which compares the
effects of PMR and short-stroked partitions:
http://www.supremelaw.org/systems/io.tests/platter.transfer.crossover.graphs.2.jpg
Not only does PMR allow greater data density on any given track,
it also allows tracks to be much closer together:
this explains the wide differences in the slopes of those graphs.
And, it should be immediately apparent that short-stroked partitions
will necessarily perform much better over time:
just compare track-to-track access times
with “full-stroke” access times in the factory’s specs.
This property of rotating HDDs is easily explained:
the recording density on each track must remain
relatively constant from outermost to innermost tracks,
in order to simplify the logic of the magnetic read heads.
Thus, there is necessarily much less data on the
innermost tracks, given a relatively constant recording
density.
And, the amount of raw data on any given track
is directly proportional to track circumference,
computed as Pi x Diameter, or 2 x Pi x Radius.
You can measure and graph this property yourself,
using the popular HDTune software / Benchmark tab.
I hope this helps.
MRFS
