TL;DR
The Codex usage reset finally arrived, and I figured I could work as much as I wanted. Instead, Codex couldn't even start a new thread. The screen just said No space left on device (os error 28).
The cause wasn't the AI, the cloud, or a rate limit. My Mac's SSD was genuinely full.
Once I deleted some junk, everything worked again. What I took away: when you work with AI tools, you have two separate fuel tanks, your usage allowance and your local storage. And before you buy more SSD space, it's well worth doing 2S first. That's "sort and set in order," the first two steps of the 5S method Japanese factories use to keep workplaces tidy.
Codex: "Your usage is back!"
Mac: "Free space: zero!"
Codex: "I just need a few KB for a lock file..."
Mac: "No room."
Me: "I can't do anything."
1. The usage reset party
On August 31, 2026, an announcement about a Codex / ChatGPT Work usage reset was making the rounds on X. The post said the reset would happen at 6 p.m. Pacific Time, and the mood was basically a festival: use up as much as you can and try the newest features.
The time difference is confusing from Japan. The US West Coast is normally on PDT (UTC-7) in August, so 6 p.m. local is around 10 a.m. the next day in Japan. If you literally calculate with PST (UTC-8), it's around 11 a.m. Either way, the reset had already happened by late morning in Japan.
OpenAI's official help pages also explain that eligible users may get a one-time reset of their Codex / ChatGPT Work usage limits. A separate system, the "banked reset," refreshes both the 5-hour window and the weekly window when you use a full reset.
So expectations that day were high.
"Okay, tank full. I can go all day."
Moments later, I found out a different tank was empty.
2. And yet Codex could do absolutely nothing
After the reset, I opened Codex to get to work, but it couldn't initialize a new thread.
This was the heart of the error:
failed to open thread writer lock ...
No space left on device (os error 28)
The important thing here is that a thread writer lock is not a big file.
Roughly speaking, a lock file is a sign that says "I'm writing this data right now." It's a tiny management file that stops multiple processes from corrupting the same conversation or state at the same time.
But when free storage drops close to zero, the system can't even put down that tiny sign.
So the reason a top-tier AI stopped was:
"There's no parking space for a few KB."
It's futuristic and small-town homeowners' association at the same time.
3. What was os error 28 actually telling me?
No space left on device means pretty much what it says: the device has no free space.
In this case, my Mac's storage really had run out. When I deleted unneeded files and freed up some room, Codex came back right away.
So the diagnosis was very clean:
- Codex tries to write a file for the thread
- The OS refuses because there's no free space
- Codex fails to initialize
- I delete unneeded data
- Writing becomes possible again
- Instant recovery
Even a cloud AI loses in the end to the physical free space on your local disk.
It's obvious when you think about it, but easy to forget when you use cloud services every day.
4. A full SSD causes more than "can't save"
Running out of disk space isn't only a problem when you save a big video.
Apps constantly write huge numbers of small files in the background.
- Lock files
- Temporary files
- Logs
- Caches
- Settings and state data
- Database updates
- Git objects and working data
- Build outputs
- Temporary space used by the OS and apps
So when free space dries up completely, even something that looks like "just a chat screen" can fail to launch or fail to create a new thread.
Apple itself says that when your Mac's startup disk is nearly full, you should check usage under System Settings → General → Storage, then free up space by deleting files you don't need, moving files to external storage, emptying the Trash, and so on.
5. Your usage allowance and your SSD space are completely separate accounts
This is the most interesting part of the whole thing.
Codex has at least two resource limits.
| Resource | What happens when it runs out? | This time |
|---|---|---|
| Codex / Work usage allowance | Running models gets restricted | Reset and restored |
| Mac's local storage | Can't write state, logs, locks, etc. | Almost zero |
In other words:
Cloud side: full tank
Local side: out of gas
It's like being handed a car with a full tank of gas, only to find the parking lot exit has been blocked with concrete.
Getting your usage back doesn't restore your computer's RAM, SSD, network, permissions, or Git state.
The more you run AI agents day to day, the more you need to watch these separate tanks one by one.
6. I freed up space, and it was fixed on the spot
The actual fix was simple.
Delete data I don't need and make some free space.
That alone brought Codex back.
That repeatability matters. If you have tens of GB free and still get the same error, you should suspect another cause. But here it was "storage really is gone, so delete, and it works again," so the cause and the fix lined up in a straight line.
This is where the idea of buying an SSD right away comes up.
But SSDs aren't cheap.
And the answer I landed on came from, of all places, the manufacturing world.
Let's do 2S first.
7. 2S before buying an SSD: the digital red-tag campaign
2S means the first two of the "5S" workplace-tidying steps used in Japanese factories.
- Sort (Seiri): Separate what you need from what you don't, and throw away what you don't
- Set in order (Seiton): Put what you need where you can grab it the moment you need it
It's exactly the same on a computer.
When you run low on space, the reflex is often "guess I'll buy a 1TB SSD." But if 100 GB of junk is sleeping in your storage, buying an SSD is like building an extension on your warehouse because the warehouse got messy.
Start with the red tags. (In a factory, a red tag is a label stuck on items that might be unneeded, so people can review them before throwing them out.)
Where to look for digital 2S
- Old ZIP, DMG, and installer files left in
Downloads - Original video and image files you only used once
- Old Git clones and duplicate repos
- Build outputs you're done with
- Unneeded images, containers, and volumes, if you use Docker
- Unneeded Simulators and DerivedData, if you use Xcode
- Old iPhone / iPad backups
- Bloated logs and crash reports
- Leftover data from apps you've already deleted
- The Trash
That said, deleting every system folder you don't understand just because it's big isn't 2S, it's demolition by explosives.
In particular, if a folder might contain development settings, credentials, or sessions, find out what the data is before you touch it.
8. "Is 1TB enough?" is easier to answer after 2S
If you buy an external SSD, is 1TB enough?
That depends on your usage, but the order in which you decide matters.
- First, delete what you don't need
- Measure how many GB you use with only the data you truly need
- Watch how much it grows over one to two months
- If your internal SSD is still constantly packed, then think about adding more
If you mostly work with articles, code, and Git, and you don't keep lots of 4K video, virtual machines, or local LLMs, adding a 1TB external drive is likely to give you a big cushion.
On the other hand, if 2S frees up several hundred GB,
SSD purchase: postponed
Wallet: survives
could well be the outcome.
Measuring before you buy is a proper improvement habit too.
9. Preventing a repeat: even a Mac needs equipment maintenance
If you've hit 0 GB once, next time you'll want to avoid running it to the limit.
- Check "System Settings → General → Storage" once a month
- Clean out the Downloads folder regularly
- Check large folders sorted by size
- Don't leave throwaway clones or generated files lying around
- Don't treat the Trash as a "warehouse of things I think I deleted"
- Back up important data before tidying up
- Always leave some headroom on the internal SSD
"Free space" is not inventory you can use down to the last 1 GB.
It's better to think of some headroom as the workspace the OS and apps need to do their jobs.
If you stack materials all the way into the aisles of a factory, nobody can walk through.
My Mac was the same.
10. Conclusion: 2S works in the age of AI too
If I sum up this whole incident in one line:
My Codex usage was fully restored, but my SSD had 0 free space, so it couldn't even create a few-KB lock file and everything stopped. I deleted the junk and it worked again.
And the lesson turned out to be surprisingly old-school.
- Check your storage
- Read the error message
- Throw away what you don't need
- Organize what you do need
- Leave some headroom
The concept that finally surfaced while troubleshooting cutting-edge AI was 2S.
Technology moves forward, and now and then it comes home to the factory.
Codex: "I can reason with the latest model."
Mac: "But I have 0 GB free."
2S: "I'll put a red tag on that."
Sources
- OpenAI Help Center — Using Codex with your ChatGPT plan
https://help.openai.com/en/articles/11369540-using-codex-with-your-chatgpt-plan - OpenAI Help Center — How banked Codex resets work
https://help.openai.com/en/articles/20001498-how-banked-codex-resets-work - Apple Support — Free up storage space on Mac
https://support.apple.com/en-us/102624 - X post referenced in the episode
https://x.com/thsottiaux/status/2094144275957350900
