MacBook Air won't start up due to full system disk

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MacBook Air won't start up due to full system disk

Mac Air Disc 2/3 full with system stuff; won't start up. Late 24 using Sequoia 15.5. Happened on a ship where I could not download to update system. Kept restarting with indications that disc is too full. Deleted everything I could and emptied trash. I do not have huge files of photos/videos etc, just documents (about ⅓ of disc). Got it working again with Safe Mode startup.

Got home and attempted to backup and same thing happening again, but Safe Mode is not successful. Tried recovery mode, disk utility repair (very quick and seemingly nothing major). Could not replace OS because not enough space.

Also got a recovery log (or whatever) onto my backup device. I do have a second functional Mac Air that is running Sequoia 15.6.

On advice of this forum I had ditched Clean My Mac a few months ago, and I expect junk was accumulating; so only had a tiny amount of disc space (about 27 MB? maybe---terrible at numbers) remaining. Was getting space error messages for a day or so before this happened, (while using Chrome), and removed practically everything that popped up.

I have a fairly recent backup on an external device, but can't turn the 24 Mac on to use it for recovery.

Anything else I can do or try? Or just go to genius? Thanks for any ideas/help.

Posted on Dec 30, 2025 1:49 PM

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Answer 1

Top-ranking reply

Tesserax

Level 10

158,058 points

Dec 30, 2025 5:50 PM in response to lbandrew

lbandrew wrote:

Are there instructions somewhere on how to boot up a computer from an external drive?

Yes, there are: How to use an external storage device as a Mac startup disk - Apple Support

Alternately (since you have a Silicon-based Mac,) if you have, or can borrow, another Mac, you can use Share Disk mode to access your MBA's internal drive from the other Mac.

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Answer 2

Dec 30, 2025 6:39 PM in response to lbandrew

lbandrew wrote:

Are there instructions somewhere on how to boot up a computer from an external drive? Won't there still be the problem of not enough room on the MBA to download it?

I took it to a "genius" who didn't have many ideas except (nuclear?) to wipe and replace. And he had to ask "his technician" to even get that idea.

When you boot from the external drive, all of the critical ops will be taking place within the macOS on that startup drive. Your internal drive will appear to be just another storage device, and you should be able to access, copy off and delete the files from it. If you can copy off and delete 100 GB or so then you should be in good shape to try to boot the computer again normally.

Our friend @Tesserax has provided a link to docs regarding starting up from external storage. 👍🏽

They also suggested the possibility of transferring files from the problem Mac by using target disk mode and your second Mac. Guidance for that is here: Transfer files between two Mac computers using target disk mode - Apple Support

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Answer 3

Dec 30, 2025 2:40 PM in response to lbandrew

Use the second MacBook Air and install macOS Sequoia on an external drive. Use that drive to boot the problem Mac and then cull more of your stuff from the internal drive. Once you've removed enough stuff, you should be able to restart the computer from its internal drive again.

I suspect you have 256 GB startup drive on the troublesome MBA?

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Answer 4

lbandrew Author

Level 1

28 points

Dec 30, 2025 5:07 PM in response to D.I. Johnson

Are there instructions somewhere on how to boot up a computer from an external drive? Won't there still be the problem of not enough room on the MBA to download it?

I took it to a "genius" who didn't have many ideas except (nuclear?) to wipe and replace. And he had to ask "his technician" to even get that idea.

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Answer 5

lbandrew Author

Level 1

28 points

Dec 30, 2025 3:11 PM in response to D.I. Johnson

Thanks. I think I have more than that, maybe 490? Ship people were amazed at how much of the space was full with system stiuff

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