Admittedly, I have had little experience with 2.5" drives since the two laptops I have owned never were upgraded. I didn't realize they only used a 5v rail as opposed to 12v for standard 3.5" drives.
That is even more reason why they could/should of at least allowed a internal drive provision.
Copied form a Fujitsu white paper on 2.5" drives;
Using a 5V power supply, the latest 2.5" HDD consumes just .5 Watts while idle,
nearly 10 times fewer Watts than a 3.5" HDD. In read/write mode, the 2.5" HDD requires only
1.8 Watts, while a 3.5" drive consumes 11 Watts. This low-power supply module also reduces
manufacturing costs, while appealing to end-users as an energy-saving alternative.
Cost savings from a smaller HDD can also be found in the drastic heat generation cuts—running
up to 30 percent cooler than 3.5" HDDs (See Figure 2). The 2.5" HDD also operates at only 20
decibels while the typical 3.5" HDD is three times louder. Larger 3.5 HDDs require a fan, which
is another contributor to noise generation, so not only does the 2.5” HDD remove the ceaseless
hum from the living room DVR, it negates the need of a fan, further reducing the overall size of
the device. Finally, 2.5" HDDs have three times the shock tolerance of their bigger counterparts,