Anatomy of a Hard Drive


There's a lot of tech that we don't understand, and consequently, a lot of tech that we don't need to understand. Do I need to know how a Hadron Collider works? Probably not. Do I need to understand that the CD tray in my desktop computer is not a coffee cup holder? Probably.

Most of us have an innate curiosity—developed when we were wee lads and lassies and thought that refrigerator gnomes existed to turn the fridge lights on and off—that spurs our need to know things. Perfect example: hard drives. We know they exist, we see them listed in the specifications for computers we buy, and yet very few know how they work. Here's a very quick look at what is inside a hard drive.

The Basics

Hard drives are composed of platters; think of them as mini vinyl records stacked on top of each other. Each record holds immense amounts of data, and to read that data (stored on the surface in tracks, like the grooves on a vinyl record, and sectors, which are pie-shaped wedges on the platter), you have to have a needle, or in hard-drive terms, a read/write head. The platter spins, the head reads and writes and data that is magically transferred from your disk to your computer. In the simplest terms, that's it.

Inside a Hard Drive

1. Platter - This is the vinyl record, so to speak; the disk that contains the sectors and tracks (all the info).

2. Spindle - This is the core that runs the platter and determines the speed of the spinning, measured in revolutions per minute (rpm).

3. Head - This is the "needle" that reads the data from the platter. When this head is knocked off balance or fluctuates, your drive crashes.

4. Arm - This holds the heads in place, and moves them around the platters when the disk is seeking a track or sector.

5. Actuator Axis - This holds the arm in place—and is another component that, if disturbed, could cause a drive failure. 

6.-8. Connections - On older drives, this is a multi-pin connector, next to a jumper port (remember the days when you used to have to set the drive to slave or master? That, and Confederate flags, are a thing of the past). With newer SATA drives, this is reduced to the SATA power outlet and the SATA data outlet.

 9. Actuator - This is the device used to move the actuator arm to different positions to read from the disk. Because it changes the head from track to track, it is the only part in the disk besides the platter that moves actively—changing heads is done electronically, and waiting for the right sector to show up under the head is done passively.


All well and good, no? But you also need to understand some of the terms when buying hard drives and, for that, we have a mini-glossary to help you out.

Access time  The amount of time it takes to access data from your storage device. This includes a combination of seek time, latency, and controller time. Shorter access time means faster access time. Look for:

Active partition  This is the partition (see below) that houses the operating system. There's only one per drive, so if you've partitioned your drive 26 times, only the primary partition can be the active one.

Cache  When you see that a drive has an 8MB cache (or similar) it means there is 8MB of RAM in the disk, set aside to help you access certain information more quickly. Every time you read or write to the disk, a copy of the information is stored in the RAM cache, along with the location on the disk of where that information came from or is going. The next time you access information, it checks the RAM cache to see if it's there and goes to it immediately. Larger RAM cache = improved performance.

Capacity  How much information can your disk hold? Simply put, a megabyte of data is less than a gigabyte of data, which is less than a terabyte of data. Therefore, a 500MB hard drive is small, a 500GB hard drive is bigger and a 500TB hard drive is ridiculous (and has not yet been achieved for consumer use).

Crash  Goodbye hard drive, hello heartache. When your drive crashes, it can sometimes be recovered through software or with a professional data recovery service. Always back up your hard drive.

Enterprise  Captain Kirk's ride. Also, any drive that is made for high-volume, 24/7 large scale operation. Buying an enterprise-level drive is a choice for IT professionals with large servers to care for. Probably not something you need in your average family desktop.

Format  The act of preparing a disk for storage by setting up the file system. Formatting erases all data on the drive.

Jumper  This applied only to older drives using EIDE connections. A jumper was a small clip you placed over certain pins in the jumper block to allow a designation for that drive. In the old days, a hard drive had to be set as a master drive, or a slave drive. A jumper was the way to do it. On SATA drives, all the assignations for drives are done through the SATA interface—no need for jumpers.

Latency  So you're spinning vinyl records, but you don't want to lift the needle to find a particular track, so you wait until the record revolves around the turntable to find your music. Same thing with your read/write heads on the hard disk. Latency is the time it takes the disk to revolve to find your data block. Average latency for a 5400 rpm drive is about 5.8 milliseconds. Average time for you to get your groove on: at least 15 minutes.

MTBF  Mean Time Between Failure, or the number of hours it takes for a drive to fail. Don't be fooled by this number; the MTBF for some drives is 5 million hours. Does that mean that your drive will fail roughly around the advent of the next Ice Age? No, it means they took 500,000 hard drives, ran them for 1,000 hours, and determined what the fail point was. A higher MTBF may put your mind at ease. But there are caveats galore to this number.

RPM  Revolutions per minute, or how many times your hard drive revolves its platter in a minute. Higher revolution means faster hard drive—faster hard drive means more heat and movement. More heat and vibration means back up your data. Generally speaking, you'll always want a drive with a higher rpm speed.

Seek time The time it takes for your hard drive to move the read/write head into the correct position. Measured in milliseconds, the lower the seek time, the faster the drive. Theoretically.

Transfer rate There are two transfer rates, in fact, the regular transfer rate refers to the rate at which the drive transmits data to the controller. This includes head repositioning, seek time, rpm—it's the total time needed. Burst transfer rate only refers to the transfer speed between the hard disk and the RAM cache.

Don’t forget to check out all of the hard drives and storage solutions available at B&H. Click this link for more information.