I’ve long been an enthusiastic user of Fusion Drives. It makes so much sense that the data your Mac wants most should be kept on faster-access media, and it seems silly to pay the higher price for SSD capacity to store lots of files which are seldom even read.

Now that SSD prices have fallen, and look to continue to fall at least in the medium term, the cost of larger SSDs of 1 TB or more is no longer prohibitive. Fusion Drives would still save you money when configuring a new iMac, for example, but the difference between built-in 1 TB options is down to around $/€/£ 630, and if you could live with a 512 GB SSD instead of 1 TB Fusion Drive, that falls to less than $/€/£ 300.

This article proposes another factor which you should take into account when making that choice: that an SSD should last longer than a similar-sized Fusion Drive. Let me take you through the figures.

The two main limitations on the life of an SSD are that all flash memory fails to retain its data after about ten years, and that it can only be written to a certain number of times. To prevent some blocks of memory from wearing out long before others, macOS and SSD firmware use wear levelling to ensure that each block of memory receives a roughly equal number of write operations, which evens out its ageing. This is combined with TRIM, which ensures that blocks of memory no longer needed for storage are returned for re-use. In this article, I will assume that wear levelling and TRIM both work perfectly, although in practice they won’t always be so good, which will shorten the life of an SSD.

Fusion Drives are managed by Apple’s CoreStorage software, which puts the files which are most frequently accessed onto the SSD, as far as possible, leaving those which are less active on the companion hard disk. It is this strategy which allows Fusion Drives to deliver performance which is almost as good as an SSD, but which also makes them age faster than an SSD alone.

For the sake of this example, I’ll assume that the SSDs involved will take 10,000 writes to any given block of memory before that block fails. That figure is arbitrary, and so long as it is uniform across the SSDs being compared is relatively unimportant at this stage.

Thus, any given SSD will fail totally when a total of 10,000 times its capacity has been written to it. So the 120 GB SSD in this iMac’s 2 TB Fusion Drive would fail completely when (10,000 x 120) GB = 1,200 TB has been written to it. In practice, an SSD is normally considered to be failing when it has reached half that total, so that 120 GB SSD should remain fine until 600 TB of data has been written to it.

Most hard drives don’t keep a reliable record of how much data has been written to them, so over the 33 months that I have been using this iMac, I don’t know the total data written to its Fusion Drive. But if CoreStorage has been doing its job properly, around 80-90% of the writes should have been to the SSD rather than the hard disk in the Fusion Drive.

Good drive utilities such as DriveDx can read the total data which has been written to the SSD; in my case, over that period of almost three years, it is nearly 21 TB. That probably represents around 25 TB of data written to the Fusion Drive’s combination of SSD and hard disk.

If I carry on using my Fusion Drive at a similar pace, the SSD in it is likely to reach 50% of life remaining in a total of 28 years. That may seem a very long time yet, and is well beyond the ten year lifespan of flash memory in any case. But it is also very optimistic: a more direct measure of expected lifespan is SMART indicator 173, the Wear Levelling Count. That currently sits at 74%, suggesting that the SSD in my Fusion Drive is likely to last another three years or so.

What if, instead of using a 2 TB Fusion Drive, my iMac had instead been fitted with a 1 TB SSD? In that case, it would have an expected total life capacity of 5,000 TB. At around 25 TB every three years, that would take over half a millenium.

In reality, with the 120 GB SSD in this Fusion Drive, I’m probably looking at a lifespan of 6-10 years; with a pure SSD, the limit is around ten years.

But what if I were using a different Fusion Drive with a smaller SSD? Some of Apple’s 1 TB Fusion Drives have had SSDs as small as 24 GB, giving a total lifetime write capacity of only 120 TB. At its present rate of writing, this iMac could take that down to a Wear Levelling Count of less than 50% in less than five years.

By making the files stored on the SSD those which are most active, CoreStorage also shortens the life of that SSD when compared with a pure SSD equivalent. That is inevitable, but does it matter: aren’t we expecting SSD lifetimes limited not by the number of writes, but by the ten year lifetime of flash memory?

Maybe not. The figures above start from the assumption that it takes 10,000 writes to any given block of memory before that block fails. Some flash used in SSDs only achieves a tenth of that, 1,000 writes, before it becomes likely to fail. When wear levelling and TRIM are less than perfect, there will be greater variation between blocks in the same SSD. The software which you use all the time could thrash large cache files which CoreStorage keeps on the SSD, and your Mac could write ten or even a hundred times more data to that SSD than mine does.

But all other things being equal, a 512 GB or 1 TB SSD is going to last much longer (up to its ten year limit) than any 120 GB or smaller SSD in a Fusion Drive. Then there’s always the possibility of the hard disk failing instead.