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Mechanical hard disk selection

Western Digital 4TB WD Red NAS Internal Hard Drive HDD - 5400 RPM, SATA 6 Gb/s

Choose the correct interface.

Choose a PATA drive if you are repairing or upgrading an older system that lacks SATA interfaces. Choose an SATA drive if you are repairing or upgrading a system that has SATA interfaces. Many hard drives are available in your choice of PATA or SATA interface, often with nearly identical model numbers. The drives may differ in appearance, but often the only obvious differences may be the data and power connectors, shown in Figure. More significant differences between models may exist. For example, the SATA model may have a faster seek time, a larger buffer, and support for SATA-only features such as NCQ.

Block Image

Figure: Two Seagate hard drives, with PATA (top) and SATA interfaces

Buy the right capacity drive.

It's tempting to buy the largest capacity drive available, but that's not always the best decision. Very large drives often cost more per gigabyte than midsize drives, and the largest drives may have slower mechanisms than midsize drives. In general, decide what performance level you need and are willing to pay for, and then buy a drive that meets those performance requirements, choosing the model based on its cost per gigabyte. Conversely, if you need a massive amount of disk storage or are implementing RAID, it may make sense to buy the largest drives available despite their high cost per gigabyte and slower performance, simply to conserve drive bays and interface connections.

Get a model with large cache if it doesn't cost much more.

Disk drives use cache (or buffer) memory to increase performance. All other things being equal, the larger the cache, the faster the performance. Inexpensive drives typically have a 2 MB cache, mainstream models an 8 MB cache, and high-performance models a 16 MB cache. Some manufacturers sell the same model drive with differing amounts of cache, often indicated by a different letter on the end of the model number. In our experience, larger caches have a relatively small impact on overall drive performance, and are not worth paying much for. For example, given otherwise identical drives, one with 2 MB cache and the other with 8 MB or one with 8 MB cache and the other with 16 MB, we might pay $5 or $10 more for the model with the larger cache.

Pay attention to power consumption and noise level.

Similar drives can differ significantly in power consumption and noise level. A drive that consumes more power also produces more heat, which contributes indirectly to overall system noise level because the system exhaust fans must work harder. For quiet system operation, it's important to use quiet, low-power hard drives. The power consumption and noise level of a drive are listed in the technical specification sheets available on its web site.

Here are some things that you can safely ignore when shopping for a drive:

Length of warranty

In late 2002, every major drive maker except Samsung reduced their standard warranties from three or five years to one year. All of the mainstream drive makers have returned to offering three-year warranties on their desktop drives, and Seagate offers five-year warranties. In practical terms, the difference is nil. A drive that is four or five years old is due for replacement anyway.


Mean Time Between Failures (MTBF) is a technical measure of the expected reliability of a device. All modern drives have extremely large MTBF ratings, often 50 years or more. That doesn't mean that the drive you buy will last 50 years. It does mean that any drive you buy will probably run for years (although some drives fail the day they are installed). The truth is that most hard drives nowadays are replaced not because they fail, but because they are no longer large enough. Ignore MTBF when you're shopping for a drive.


Mean Time To Repair (MTTR) is another measure that has little application in the real world. MTTR specifies the average time required to repair a drive. Since nobody except companies that salvage data from dead drives actually repairs drives nowadays, you can ignore MTTR.

Shock rating

Drives are rated in gravities (G) for the level of shock they can withstand in both operating and non-operating modes. For drives used in desktop systems, at least, you can ignore shock rating. All modern drives are remarkably resistant to damage if dropped, but all of them break if you drop them hard enough.

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