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

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.

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

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?

My daughter's overly-full Intel iMac 2019 (with 512GB SSD option) would not start for her. By using Safe Mode, I was able to get a usable desktop. From there I discover that, in addition to 70GB assigned to a shared user, whe had 250+GB of images because she "did not trust" iCloud storage.

See this: Start up your Mac in safe mode - Apple Support

Ship people were amazed at how much of the space was full with system stuff

There are also a lot of reports here of System files and "Other" getting bloated. That can be a different issue. Obvioulsy , CleanMyWallet, oops! I mean CleanMyMac did nothing fo you. The easy check is to use Disk Utility (in Applications > Utilities) to see if you have a bloat of APFS snapshots:

One senior contributor found 36 GB of them on one computer. Delete all but the that two or three.

I think it was our wise Owl-53 who reported this.

When you download the Sequoia installer from the App Store, the application "Install macOS Sequoia.app" gets deposited into the Applications folder of your startup drive. If the app launches immediately after downloading simply quit the installer app (⌘Q). The install app does not get copied to the external drive.

To format the external drive, launch Disk Utility. In the Disk Utility menu bar select View > Show All Devices. You should now find the external drive listed in the DU sidebar. It will be named something like Drive Brand... Media. It is this media device that you must select and erase. Use the GUID partition scheme, APFS format and whatever name you like.

With that done you can launch the macOS installer found in the Applications folder, and when prompted, select your freshly erased external drive as the install destination.