Investing in a Hard Drive

Investing in a hard drive requires some knowledge on the part of the customer and the installer. There are five basic criteria which define the efficiency and value of hard drives. They are:

Rotational speed
Access time
Data Transfer Rate
Data storage size
Interface

Rotational speed is one of the main bottlenecks in processing speed. This refers to how fast the disk is spinning in relationship to the heads. It is sometimes referred to as “spindle speed.” Most home computer systems using conventional PATA or SATA interfaces rotate at 7200 RPM. Older drives rotate at 5400 RPM. The fastest commercially available hard drives rotate at 10000 RPM. This is a 30% faster drive than a 7200 RPM drive and retrieves or stores data that much faster. These drives are generally small capacity, however, the are outstanding for operating system files, saving a lot of waiting-time overhead. SCSI drives can have rotational speeds in excess of 20000 RPM.

Access time is the characteristic of a hard disk drive to put the read/write heads on top of the data to be transferred. The lower the number in milliseconds, the better. The 4.x millisecond range is good. Massive storage drives in excess of 1.5TB can run access times in the 8.x ms range.

Data transfer rate refers to how much data can be retrieved from the hard drive per second. SATA 3.0 is able to take 3GB of contiguous data off the drive per second, double the amount of SATA 1.5. A closely related characteristic of hard drives is how much of that data can be cached for “look-ahead” anticipatory data retrieval. 2MB isn't enough, 16MB is better, and 32MB is much more preferable.

Storage is always an issue. The total storage of a device is actually less than the formatted capacity. Up to 10% of the disk space can be “wasted” on formatting tables. The basic rule of thumb for determining the value of storage is to calculate the cost per GB. The lower the ratio, the better. Typically, 10 cents per GB is about average for a 640GB 7200 RPM SATA 3.0 hard drive.

The hard drive interface is how the data gets from the on-board disk electronics through a plug to the motherboard or hard drive disk controller. The most common interface on modern machines – made in the last four years or so, is the SATA. This is the Serial ATA data transfer standard and is immediately recognizable by the thin, narrow and flat data cable with only two small connectors. This is the fastest and easiest way to connect the hard drive to a compatible motherboard or hard disk controller. The legacy PATA cables are wide, flat ribbons, typically with three large wide connectors. The SCSI interface is usually only seen on servers or high-end home machines. If the computer has a SCSI set-up for the hard drives, it is beyond the scope of this article.

Buying an affordable hard drive is not complicated, however, it is necessary to review the technical specifications of the hard drives being considered to ensure the best value.