molinger wrote:
Two year old MacBook M3 with 1T drive. System data is taking up 668G...two thirds. Deleting items causes it to go up and so do not have enough space on the drive to download software to investigate or reinstall OS. All remedies I find do no help. Looking for ideas, thanks.
Try restarting your Mac in Safe Mode, as described here: Start up your Mac in safe mode - Apple Support
This will prevent most 3rd party system modifications from running. The last two people that I've seen report this problem have discovered that the Carbonite software was causing it. So Safe Mode should prevent that software from running and give you a chance to get ahead of it and delete some files.
After deleting files, it may take a day or so before the system finally cleans out the local snapshots that are holding those deleted files. You may be able to force the process by using Disk Utility > View > Show All Devices and then Show APFS Snapshots. Deleting those snapshots should give you back some space.
But all of this is merely to get ahead of the problem. Once you restart normally, it will likely return with a vengeance. You have to identify what software is causing it. I only mentioned Carbonite because two people recently identified it. But it could be literally anything. It could even be part of the operating system and Safe Mode might not help. But it can't hurt to get lucky!