A Look at Your Hard Drive . . .
Melanie McDonald
A funny thing happened to me on the way to a sick G5. A client called and asked me what goes on in the hard drive and what causes occasional problems with these drives.
Since this is a question I am frequently asked, I thought you might like to know the answer, too.
A computer’s hard drive is made up of two or more spinning platters made of aluminum, glass, or ceramic and coated on both sides with a magnetic material which contains particles of iron or metals oxide. (
The larger the hard drive the more platters it has.) On these magnetized areas, the 1s and 0s of binary information are written in the same way sound is recorded on tape.
These, separated by disk spacers, are clamped to a rotating spindle that turns all the disks in unison at a set rate as high as 7,200 revolutions per minute. At the same time small electromagnets called heads are moving back and forth over the surface of the disks writing and reading data. With one head active at any given moment, the heads not in use are held suspended over their respective disks by the air pressure created by
the spinning disks. Because the distance between a head and its platter is only several dozen microns, it is worth noting that a single speck of dust or even a fingerprint left during the assembly of the drive is sufficient to interfere with the spin of the disk.
This platter and head complex is attached to a circuit board that holds the controlling electronics for the drive. This total hard drive mechanism may be inside the Macintosh or cased as an external hard drive.
Before this drive will accept data, it must be formatted either at the factory or by the user with Apple software that comes with the machine. The formatting process, the Hierarchical File System Extended (HFS), leaves an organizational grid to hold data on the disks; it also creates information on the disk that is loaded into the computer’s RAM at startup.
As you can see from the description of the speed and precision of the disk movements, a jog while the hard drive is running or a dirty drive can create considerable trouble. For example, back in the days when people smoked in offices, I had a client who kept his ashtray near his computer fan. The cigarettes he constantly left burning in the ashtray while he worked finally turned his hard drive from a clean, efficient, and brisk bit of electronics to a black leaden mess.
Now let’s take a really brief look at what happens in your hard drive when you send it a command to open a program or record data. The command is received and processed by the drive’s controller circuitry on the logic board. If you send a write command, that data is stored in the write cache and tells the processor the
task has been completed. Your logic board talks to the head actuator, increasing current sending the heads toward the center of the disk and decreasing current sending them to the outer edge of the disk.
Simultaneously these heads note the heavily magnetized lines that mark sector boundaries and count the boundaries to determine the correct sector for them to be in as a result of your command. Then the head is over the appropriate sector and, in a series of magnetizing maneuvers, creates areas with polar reversals accommodating the binary 1s and 0s. At this point the volume bitmap in your RAM locates the free blocks on the4 disks into which your data can be written. If it is necessary to write this data in several non-contiguous blocks, your disk is said to be fragmented. A fragmented disk can slow down your drive since the heads must wait for the disk to spin farther from block to block when reading the file.
Now if you are retrieving data—opening a program or a document—you have the reverse of the situation just described: The heads move to the relevant blocks and the data retrieved is put into the drive cache.
All of the above may be more than you ever wanted to know about your Mac hard drive, but it does suggest that a bit of hard disk housekeeping as you go along may go a long way to keep your delicate disks whizzing along happily. For instance, some good defragging software like Micromat’s Tech Tool Pro can be used to determine the degree of fragmentation on your hard drive and, when that percentage passed 30 or so, can defragment your disk. This same software package will run diagnostics on your hard drive and perform some simple clean-up procedures when it encounters problems with hard drive organization and data.
McDonald, president and CEO of MacTutor & Services in Southern Pines, is an apple authorized service provider whose business includes technical and networking services as well as system and software tutoring and design consulting. She can be reached at 910.246.2150 or mobbs@pinehurst.net.
