HD or SSD?

Most of the computer-like devices that you use now store their data on solid-state disk (SSD) rather than traditional hard disk (HD) components. Are they also the best way ahead for regular computers?

Only desktop Macs still spin traditional disk platters, but all others – MacBook, MacBook Pro, MacBook Air, iPod, iPhone, iPad, and even the Mac Pro – rely on electronic memory, no moving parts involved. Although capacious SSD storage remains expensive, perhaps the time has come to replace at least some hard disks with SSD.

You would not be alone in doing so. As SSD costs have fallen, and capacities risen, they have become popular in demanding situations such as servers, for which EFD (Enterprise Flash Drive) has been coined.

Lower power consumption and heat output reduce cost and requirements of power supplies, UPS (uninterruptible power supply) capacity, air conditioning, and more. SSDs are also claimed to have lower failure rates, and can perform better than even enterprise-quality hard drives. So apart from initial cost, SSDs would appear to win across the board. If only the comparison was that simple!

Hard disks use well-proven technology, which first appeared in the original ‘Winchester’ drives nearly 60 years ago. By the mid-eighties, PCs came with optional 10 or 20 MB hard disks, and the first Mac with an internal HD was the SE of 1987. By the 1990s, the first gigabyte hard drives were appearing, with 250 GB options available for the Power Mac G5 of 2003 – when Apple switched from using IDE or parallel ATA interfaces to the serial ATA (SATA) type that prevails today.

Hard disks

A hard disk contains one or more rigid platters coated with ultrafine magnetic particles. A read/write head is tracked over the surface(s) of the platter, with electric motors spinning the platters at high speed, and positioning the head(s) very precisely over the platter.

Data are written to the disk by aligning those magnetic particles, and read back by detecting their alignment. Hard drives for laptops normally spin their platters more slowly, at around 5,400 rpm, than those for desktop systems, which commonly spin at 7,200 rpm, although some specialist models can reach 15,000 rpm.

There is plenty of precision engineering involved: with data tracks only tens of nanometers apart, the heads have to be positioned very accurately, keeping an air gap between head and platter of around 30-50 nm, less than one-thousandth of the thickness of a sheet of thin paper. Being highly electro-mechanical, hard disks can fail in many ways.

Among the more spectacular are true ‘crashes’, when the heads strike the platters and strip their magnetic coating. IBM’s notorious ‘deathstar’ (Deskstar) drives had a predilection to do just that, often when only a few months old, but these are thankfully rare in modern HDs.

Positioning the head correctly over a platter to seek out data requires time, and great ingenuity has been employed to improve performance, with local buffer memory and more being used. Unfortunately performance usually degrades with time, as the contents of a disk become stored in progressively more fragmented locations.

Sealed inside a metal case, the motors require significant electrical power, drawing most when spinning a disk up from rest, generate considerable heat, and are a source of noise. Such heat must be removed, so that the disk is kept relatively cool, as running hard disks at high temperatures (over 47˚C) may increase their failure rate.

Some ‘green’ drives reduce power consumption and heat output by using lower rotation speeds (such as 5,000 rpm), which may be varied according to demand. With such a fine air gap between heads and platters, HDs are not able to tolerate jolts and shock loading very well, although models for use in laptops (with a 2.5” form factor, compared with 3.5” for desktop systems) should cope better.

Hard disks remain hugely popular, with over 500 million new disks being shipped worldwide every year, and have become cheap and capacious, with 1 to 2 TB units now commonplace.


Typical prices for 1 TB models of high specification, as might be suitable for round-the-clock server or other heavy work, are around £80 inclusive of VAT. In contrast, SSDs of 1 TB and above are hard to come by, and about six times as costly. More typical SSD units in a SATA package offer 240 to 256 GB of storage for around £100.


Normal computer memory is volatile (VRAM, dynamic RAM or DRAM, or SDRAM), as it loses its contents when powered down. Such volatile memory storage is as old as the computer itself, but non-volatile (NVRAM) or ‘flash’ (technically known as NAND-based flash) devices that functioned as SSD did not start appearing in quantity until the late 1990s.

Only in the last 8 years have devices of tens or hundreds of gigabytes capacity become available at prices that bring them within reach of ordinary users; now iPods and tablets commonly offer 64 GB of storage, and 16-32 GB flash dongles and memory cards are inexpensive.

Being solid-state, and having no need for electro-mechanical seeking of heads over platters, SSD storage modules should perform far better than HDs. However writing to flash memory is considerably slower than reading from it, and mixed read-write operations slow further.

To achieve performance comparable to the fastest hard disks, various tricks are used, such as interleaving storage rather than using memory in a single linear block, and SSDs with better ‘enterprise’ performance have sophisticated controllers. Some modules designed for desktop systems, which will have power applied continuously, may use volatile memory, which is far quicker to access, particularly when writing.

SSDs contain no moving parts, generate no noise, are not susceptible to jolt or shock loads, have low power consumption, and generate much less heat than HDs. Some of the largest models currently available have cooling fans, but these are not required for storage in the popular range of around 250 GB.

Their advantages are well illustrated in the MacBook Air, which has a very long battery endurance, does not burn your thighs when rested on the lap, is surprisingly thin, and runs silent much of the time.

Because capacious SSDs are relatively recent, there is limited experience of their heavy use in servers and similar environments. Expectations that they would not have a significant failure rate until they reached the write limit of their flash memory components are not being met, with enterprise users reporting similar failure experience to that of conventional hard disks.

Getting the best out of both types of storage depends on how you use your Mac. One design that works well for demanding situations such as servers and high-end workstations is an SSD startup disk of about 250 GB capacity, on which Mac OS X and key applications are kept.

These by default will also keep their cache and working files on the SSD, where they can accelerate performance. Bulk storage, of large databases and media files, can then be on multiple 1 to 2 TB HDs, on the internal SATA bus and high-performance external buses. This is little more expensive than using conventional HDs alone, but could provide valuable enhancements to performance. Apple offers combination Fusion Drives in current iMacs.

In Use: Hard Disks

Selecting and managing hard disks to populate an old Mac Pro’s internal bays, or replace a failed drive in an iMac, can be bewildering. Two of the critical features, apart from cost and capacity, are performance and longevity.

Modern hard disks incorporate memory buffers that even out the flow of data to and from the disk, and extensive electronics to control the tracking of the heads and more, in the effort to squeeze best performance from the disk. Even so, when files become fragmented over different sectors of an HD, reading and writing to them becomes slower.

Mac OS X automatically defragments many of its files, but it does not yet rearrange the location of free disk space; fragmented free space can have substantial performance effects on scratch and other temporary storage, thus on application and system performance.

Whichever model you choose, you should ensure that a good percentage of the disk space is kept as free space. Ten percent is an absolute minimum, but 100 GB is good, and 200 or more ideal. If free space falls below about 20 GB, or the drive is used for a long period without defragmenting free space, performance may slow noticeably. You can then clone the drive to another disk and back again to join up all its existing free space and boost your Mac’s performance.

Most internal HDs now come equipped with built-in diagnostic firmware and self-diagnostics known as S.M.A.R.T., that are intended to provide early warning of imminent failure, but large-scale surveys suggest that few serious hard disk failures are preceded by helpful S.M.A.R.T. warnings. Unfortunately this is not supported over FireWire or even Thunderbolt to external disks, but it is still worth keeping an eye on S.M.A.R.T. status day to day, either in Disk Utility or a more specialist tool.

In Use: Solid-State Drives

It is easy to see how the intricate mechanisms inside a hard disk can wear out and break, but the illusion has grown that SSDs are totally reliable. This is not accurate: they have a finite working life, and although failures should be fairly uncommon, they can suffer problems long before they reach the end of their life.

Unlike VRAM, flash memory only works for a limited number of writes; this should normally far exceed the expected life of the computer or device, but requires controllers that are designed to balance out the ageing of blocks within the SSD so that no part is used excessively and fails early.

Even so, as with other electronic components, there is a small failure rate during life, which appears just as in hard disks to vary considerably by manufacturer, model, and batch. Look for no-quibble warranties that last for the expected period of use, and do not stop backing up the contents of SSDs to more permanent media.

As more SSDs have been put to use in demanding situations, it has become apparent that write performance will also degrade progressively long before that flash memory’s maximum number of writes is reached. Modern SSD controllers now incorporate the TRIM command to eliminate this problem. TRIM marks out those blocks within the SSD memory that are no longer being used, so that the controller can wipe them clean and return them to use, restoring write performance.

Support for the TRIM command in Apple-branded SSDs was included in the Mac OS X 10.6.8 update, and is a feature of Lion and later, but if you intend using a third-party SSD product check that it can be enabled, and have been updated for Yosemite (which broke most older third-party TRIM drivers). It is still not clear whether this might affect iOS devices that are heavily used, nor whether iOS yet has TRIM support.

Updated from the original, which was first published in MacUser volume 27 issue 21, 2011.