It’s not that long ago that our Macs came with internal storage that could readily be replaced when it failed. Memories of big hard disks that died almost as soon as their warranty ran out, and of keeping a bootable clone ready in a Mac Pro, aren’t easily forgotten. So isn’t it high risk to buy a modern Mac that won’t even boot if its internal SSD has failed? Are you left wondering whether that SSD will last five years, or even three?

SSDs aren’t like hard disks

Hard disks are amazingly engineered electro-mechanical devices that spin platters at high speeds incredibly close to read-write heads. Before you even consider all the faults that can occur in their magnetic storage, there are many horrible ways they can die through mechanical disaster. Visit a data recovery shop and they’ll show you heads fused to platters, and shards of what had been storing terabytes of data before the platter shattered. And like all mechanical devices they wear out physically, no matter how carefully you care for them.

By comparison, an SSD in a Mac that has good mains power filtering, ideally a proper uninterruptible power supply (UPS), leads a sheltered life. Like other solid-state devices, so long as its power supply is clean and it doesn’t get too hot, it’s most likely to fail in the first few weeks of use, and as it’s reaching the end of its working life, in a U-shaped curve. Modern quality control has greatly reduced the number of early failures, so what we’re most concerned about is how long it will be until it wears out, as it approaches its maximum number of erase-write cycles.

Predicting wear

The theory goes that the memory cells used in SSDs can only work normally for a set number of erase-write cycles. This appears to hold good in practice, although there’s always a small number that suffer unpredictable electronic failure before they reach that. What’s more controversial is how many erase-write cycles each SSD should be capable of. Manufacturers make various claims based on accelerated ageing tests, and I suspect most come with a large dash of marketing sauce. Apple doesn’t offer figures for the SSDs it equips Macs with, but conservative estimates are around 3,000 cycles in recent models.

To work out how long you can expect your Mac’s internal SSD to last before it reaches that cycle limit, all you need do is to measure how much data is written to it, and once that is 3,000 times the capacity of the SSD, you should expect it to fail through wear. Fortunately, SSDs keep track of the amount of data written to them over their lifetime. This can be accessed through better SSD utilities like DriveDx, and I even have a feature in Mints that will do that for most internal SSDs.

Example

My iMac Pro is now well over 7 years old, as it was bought new in December 2018. It has a 1 TB internal SSD (I wanted 2 TB, but couldn’t wait for a BTO), and has run pretty well 24/7 since I got it. As I work every day, even over Christmas, and it has been my main production system, it has probably been in use for over 2,500 days now.

According to the SSD’s records, over that period its 1 TB SSD has written about 150 TB in total, from its total expected lifetime of 3,000 TB, if it reaches 3,000 erase-write cycles. At current usage rates that would take another century, or 133 years if you want to be precise. In reality, it’s generally believed that most SSDs will cease functioning after about 10 years in any case.

It’s worth noting here that, had I got the iMac Pro with my preferred 2 TB SSD, its total expected lifetime would have been 6,000 TB, and instead of lasting a total of 140 years it would in theory have gone twice that period before it wore out.

What wears out SSDs?

For an SSD to wear out when it reaches its limit of erase-write cycles, wear across its memory must be even. If that memory were to be largely full of static data, and the SSD was only able to write to 10% of its memory, then it would wear out ten times quicker than the whole SSD would. To ensure that doesn’t happen, all modern SSDs incorporate wear-levelling, which incurs its own overhead in erase-write cycles, but should ensure that the whole SSD wears out at the same rate. You can help that, and maintain faster write speeds, by keeping ample storage space free. My current target for my iMac Pro is an absolute minimum of 10% free, and 15% as much as possible.

Given that my iMac Pro has averaged about 21 TB written to its SSD each year, that works out at just under 60 GB per day. For those who are worried that the Unified log adds significantly to SSD wear, it’s not hard to estimate that’s only likely to write around 250-500 MB each day even if you leave your Mac awake and running 24/7, less than 1% of my Mac’s daily write load.

Unless you work with huge media files, by far your worst enemy is swap space used for virtual memory. When the first M1 Macs were released, base models with just 8 GB of memory and 128 GB internal SSDs were most readily available, with custom builds following later. As a result, many of those who set out to assess Apple’s new Macs ended up stress-testing those with inadequate memory and storage for the tasks they ran. Many noticed rapid changes in their SSD wear indicators, and some were getting worryingly close to the end of their expected working life after just three years.

So the best way to get a long working life from your Mac’s internal SSD is to ensure that it has sufficient memory as to never use swap space in its VM volume. Although my iMac Pro only has a 1 TB internal SSD, which is more cramped than I’d like, it has 32 GB of memory, and almost never uses swap.

Key points

• SSDs wear out differently from hard disks.
• Protect your Mac and its internal SSD with good mains power filtering, preferably using a UPS.
• Expect modern Mac internal SSDs to wear out after at least 3,000 erase-write cycles.
• To monitor wear, measure the total data written to the SSD.
• Expect an internal SSD to wear out when that total reaches 3,000 times the total capacity of the SSD.
• For a given amount of data written to an SSD, the larger the total capacity of the SSD, the slower it will wear out.
• Keep at least 10% of the SSD free at all times, with 15-25% even better.
• Ensure your Mac has sufficient memory to never use VM swap space.