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| There are several key ratings that will help you to understand and evaluate the performance of hard drives. These ratings are seek time, RPM, the potential data rate of the connection format, and the most overlooked but very important number—actual read/write data rates. |
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| Seek time is the measure of how long it takes for the drive head to move to the correct sectors on the drive to retrieve or write data. It is measured in thousandths of a second (milliseconds or ms), and typically varies from 4.5ms (very fast) to 9.5ms (adequate). It is important because it determines how long it takes before the drive starts reading or writing data. |
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| The RPM rating or "revolutions per minute" is the physical rotation rate of the hard drive platters. This is the same RPM measurement you see on a car's tachometer. It is an important number because it is related to the disk drive's data rate. 5400 RPM is a relatively slow drive. Faster drives run between 7,200 RPM and 10,000 RPM. |
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| Disk drives are often advertised with ratings of 40 or 80 megabytes per second (MB/sec). This number refers to the data rate of the SCSI bus it can connect to, not to the sustained performance of the drive itself. Common potential data rates are 80 MB/sec for LVD Ultra2, 40 MB/sec for Ultra SCSI 2, and 66 MB/sec for ATA 66. |
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