electromagnetism
I noticed the other day that the rotational frequency of typical consumer grade HDD are either 5400rpm or 7200rpm. These correspond to 90Hz and 120Hz respectively.
Given that in parts of the world the grid frequency is 60Hz, and 120Hz is the first harmonic. What effects does this have on the platters.
It would seem to me that this choice in rotational speed (7200 rpm) would be problematic in these certain countries.
In the passage you quoted, McDonald is describing the very edge of the thin conducting disk. Recall that although the disk is thin, it does have a finite thickness. Therefore, its edge looks like this, with the conductor drawn in grey:
One way to describe this geometry is to say the internal angle is $\pi/2$. McDonald chooses to instead say that the exterior angle is $3\pi/2$. He does this to maintain consistency with the reference he cites (Jackson): if you look up figure 2.13 on page 78, you'll notice this angle being described as $3\pi/2$.
For crystal materials like diamond, this is a result of extremely stable bonds between the constituent atoms of the material. These bonds are only formed under immense pressure because inter-atomic repulsive forces keep the atoms from coming close enough to be forced to arrange themselves in that crystal structure.
Metals alloys like steel are much harder than elemental metals because they are a mixture of different elements like iron, nickel, chrome, etc. Basically, when a metal is composed uniformly of the same atom, they fit together very tightly and slide over each other easily. This is why pure gold is so soft and malleable.
A steel alloy, on the other hand, is composed of a few different elements. The different sized atoms mixed around in the bond structure cause irregularities in the atomic organization of the material, which makes it makes harder for slips and other deformations to occur.
Best Answer
The power supply to a hard disk is DC not AC (5V and 12V) and the disks themselves are heavily shielded from external magnetic fields - for obvious reasons! So there is no interaction, adverse or otherwise, with the mains frequency.