Hard Drive Comparison
We ended up having two hard disk drives in our possession: one Western Digital, the other a Seagate. The WD is an EIDE, single platter drive; the Seagate is a SCSI, multi-platter drive.
To aid our destructive process, we found some course notes from an undergraduate engineering course at Stanford in which on the of the lab sessions involves taking apart hard drives of two different vintages. Here are the two drives before we got started:
Incidentally, the Wikipedia entry “hard disk drive” is pretty complete.
The underside of the SCSI drive is shown below. There is a considerable amount of “stuff” crammed into these things – every square inch of real estate is used up in drive management and control electronics. Note that the electronics live on the outside of the sealed case – there is a nifty sealed connector to transfer data into and out of the case. The control for the voice coil actuator also passes through this connector. The platter spindle motor in the SCSI drive passed through this connector, but a different technique was used in the EIDE drive.
Getting the board off on the EIDE. The small orange flex circuit is the interface to the platter spindle motor drive.
In both cases, the cases were cast Al with a considerable amount of post-machining. The post-machining was used to create registration and datum features enabling alignment of critical elements like the read head bearing and the platter spindle bearing. In both cases, Torx screws were used.
A better view of the inside of the EIDE drive. The interior of these cases are not hermetically sealed, but have small “breather holes”. They are sealed from dust, air passes in and out though a carbon filter located near the breather hole. Moisture control is critical in the performance of these drives – in both cases dessicant pouches were used to absorb excess water vapor. Air pressure is also a critical parameter. Most drives will not operate above 10,000 ft. More on this later…
The write heads of these drives are actuated with a voice coil motor. A voice coil is a simple actuator – the simplest form of electro-motor. The voice coil consists of a coil, or bobbin, of looped wire. It lives between two powerful permanent magnets. The magnets generate a strong, consistent magnetic field in the gap between them. When electric current is passed through the coil, a magnetic moment is created – this moment “reacts against” the static field. The magnitude and direction of the magnetic moment is proportional to the current and its direction. When coupled with a servo control mechanism, precise positioning can be achieved. The disk itself provides an ideal position feedback (in the form of tracks and sectors) as read by the read/write heads.
The disk itself is also pretty cool. These disks are usually not magnetic in their own right. Often they are aluminum or even glass. A very thin layer (probably vacuum deposited) of magnetic alloy is added – this is the data storage layer. Modern disks use a cobalt-based alloy.
The write heads are tiny pieces of silicon that seem to contact the disk directly. In practice, they float under air pressure generated by the spinning of the disks. This spinning action has a couple of beneficial effects to internal airflow: it generates a thin film of air that allow the heads to fly about 15 micro-inches above the disk. This effect is analogous to hydro-planing of automotive tiles on films of water on the road. The second benefit are that the air currents help direct any particulates to small filter “traps” located in once corner of the case.
Close-up of the write head with voice coil and bearing shown. The heads themselves are suspended on some very elegant flexure supports. The tip itself is blurry in this view. Stupid… macro… lens…
Another beautiful picture. Note the multiple disks in this SCSI disk.
