The one-inch IBM/Hitachi Microdrive was for years the world's smallest hard-drive. We open one up to see what's inside.
1 Read/write head
One of the key components of a hard- drive, the read-write head does much the same thing as a cassette recorder, converting an electrical signal into a varying magnetic field – which is imprinted onto the magnetic surface – and then reading back that magnetic imprint. Modern drives have separate but physically very close read and write elements, typically a thin-film semiconductor write head and a magneto-resistive read head, the resistance of which changes according to the magnetic field sensed. One read-write head is needed for each platter side in use – one for a single-sided single-platter drive, say, or 16 for a high-capacity eight-platter server drive.
The arm that carries the read/write head is controlled by a motor and must be both light and very rigid: acceleration at the head in modern drives can exceed 500G. On larger drives, several actuator arms are slaved together as a stack, so all the arms move together to describe a cylinder on the platter stack. The actuator will normally move off the platter when powered down or when a shock is detected. A locking latch holds the Microdrive's actuator static when not in use, preventing it from damaging the disk surface if jarred or dropped.
3 Disk motor
The Microdrive's motor incorporates nine electromagnets or stator coils, and must spin smoothly and consistently for many thousands of hours at an energy-efficient 3,600 RPM – by comparison, high-performance server drives run at up to 15,000 RPM. Also important is how fast it can spin up to its operating speed, as many devices will turn off the drive to save power if it has not been used for a while.
A faster spin allows data to be read more quickly, but of course needs better bearings – hydrodynamic fluid bearings are now the norm, replacing the ball-bearings of earlier generations.
For many years platters were aluminium alloy, but glass platters have taken over because they are more rigid, thinner and more thermally stable. The platter has to be smooth and stable because the read/write head flies incredibly close to it – so close that a particle of dust could crash it, damaging the surface and creating debris that can cause further crashes. A smaller platter holds less data but can spin faster on less energy.
The platter's magnetic coating is applied in layers, and the more advanced the properties of the disk, the more layers are needed. The resistance of a material to being magnetised is called coercivity and is measured in Oersteds – a higher coercivity means that the magnetic tracks on the disk can be narrower as it is less likely that the signal from the write-head will mess up adjacent tracks. A complex multi-layer coating could have a coercivity of 5,000 Oe or more, yet the coating on a Microdrive's platter is less than one thousandth of the thickness of a human hair.
The drive's electronics – centred on a small PCB, as in the Microdrive – convert data into the electrical write signal, and convert the output from the read head back to digital data. Specialist software handles tasks such as optimising disk access; for example if a program requires cylinders 1, 1000 and 500, it minimises head movement by going 1, 500, 1000 instead. This is also where advanced techniques such as PRML (partial response maximum likelihood) take place – instead of merely taking a single bit sample, PRML takes samples either side too and uses these to help deduce the bit setting – along with read and write caching.
The drive is installed within an aluminium casting, normally with a dust-proof gasket and then an aluminium lid fixed by screws. The Microdrive's casing also incorporates rubber shock absorbers to help protect the device from the everyday impacts of mobile life. *