How Much Has Been Written to a Mac SSD: Checking Wear

macOS’s built-in tools show only the S.M.A.R.T. status, not data written or wear. Lifetime terabytes written and the share of endurance used come from the free smartctl tool from Homebrew — five minutes in Terminal.

Check it once — chances are you can stop worrying after that.

What macOS itself shows

System Information. Open it from Spotlight or Applications → Utilities → System Information and select NVMExpress on the left. The internal drive has an S.M.A.R.T. Status line at the bottom that reads Verified. That’s the only health indicator there — no data written, no wear percentage. Howard Oakley, who writes about macOS internals on the Eclectic Light blog, calls this entry simplistic: it doesn’t even report when the drive was last checked (his explainer).

The same in Terminal:

system_profiler SPNVMeDataType

Look for the S.M.A.R.T. status: Verified line.

Activity Monitor, Disk tab. The Bytes Written column shows how much each running process has written. It’s useful for spotting what’s writing a lot right now — but it isn’t wear, and it isn’t the drive’s lifetime history. More about the tab in the Activity Monitor guide.

How to see data written: smartctl

smartctl is part of smartmontools — free and open source. It installs with Homebrew, if you already have it:

brew install smartmontools

Find the internal drive’s identifier — it’s usually disk0:

diskutil list

You want the disk marked internal, physical. Then:

smartctl -a disk0

If it says it lacks permissions, run it again with sudo in front — it will ask for an administrator password. Both diskutil list and smartctl -a only read data and change nothing on the drive.

If Terminal isn’t for you, there are also apps with a graphical interface. The Stats and iStat Menus monitors, for example, show S.M.A.R.T. status — what else they do is in the system monitor comparison.

How to read the result

In the long output you need three lines from the SMART/Health Information section:

• Data Units Written — how much has been written over the drive’s lifetime. The first number is in units of 1,000 blocks of 512 bytes; there’s no need to convert it: smartctl prints the amount in square brackets, for example [85.2 TB]. Next to it, Data Units Read is the same for reads;
• Percentage Used — the manufacturer’s estimate of how much of the drive’s rated endurance has been used;
• Available Spare — how much of the spare capacity the drive uses to replace worn cells is left. A new drive shows 100%. Below it, Available Spare Threshold is the level under which the spare is considered critical.

Is that a lot?

Apple doesn’t publish a write endurance (TBW) rating for its internal SSDs: the MacBook Air, MacBook Pro and Mac mini specs list only the capacity. So there’s no official limit to compare “85 TB written” with — and don’t trust tables that claim to know one for your model.

Oakley offers a reference point: in his estimate, even the busiest users don’t write enough to a Mac’s internal SSD to wear it out over its normal lifetime of around ten years (a 2022 article).

It’s worth a closer look if:

• Percentage Used climbs fast. Note today’s Percentage Used and Data Units Written and compare in a month or two. If far more has piled up than you’d expect from your work, find out what’s writing — a common suspect is below;
• Available Spare drops toward the threshold — then make a fresh Time Machine backup and don’t put off service.

What adds to writes

• Swap. When memory runs short, macOS moves data out to disk. Oakley suggests looking at swap’s share of all writes: under 10% and it contributes little to wear. How to tell whether swap is growing is in the swap guide; where it comes from is in the memory pressure guide.
• Big projects. Video editing, virtual machines, building code — all of these mean gigabytes of temporary files.
• Caches of browsers and apps that are constantly refreshed. Where they live and why “cleaners” don’t help here is in the guide to app size and removal.

Don’t turn off swap, Spotlight or Time Machine to save the SSD. There’s no supported swap switch, and the workarounds weaken the Mac’s security; without Spotlight and Time Machine you lose search and backups.

External drives

External drives are trickier. As Oakley observes, macOS has never given access to S.M.A.R.T. attributes for drives connected over USB, while Thunderbolt should give access by default. If smartctl gets no data from a USB drive, that’s a limitation of the connection, not a sign that something is wrong with the drive.

In the app

To be clear up front: Mac Resources doesn’t show wear, data written or S.M.A.R.T., and it doesn’t show read and write speed either. For that, use smartctl.

What the app does offer alongside:

• the small Disk widget shows how full the startup volume is, a graph for the past hour and the amount like “2.4 of 4.0 TB”. Sizes use base 1000, as in Finder. Disk isn’t part of the menu bar item: being full isn’t load;
• the menu bar item turns red above 80% and shows the symbol of the busiest metric. Red memory is the road to swap, and so to extra writes to the SSD: a good moment to open the Memory tab in Activity Monitor.

If the disk isn’t the problem and the Mac is still slow, walk through the step-by-step diagnosis — step 3 there is about the disk.