M4 Mac mini internal drive space rapidly filling in real-time

Something you might check:

Activity Monitor.app > Disk > Bytes Written (largest at the top): is there some app that writes lots of stuff?

Is iCloud Drive set as optimized? It might take a few days to settle down. You have iCloud Photos on external drive but make sure you don't have another Photos library on the internal drive.

Check Disk Utility.app > View > Show APFS snapshots and check all volumes -- are there lots of snapshots that take space? If you use Time Machine make sure it has been given a chance to clear its snapshots.

Boot to safe mode to clear some caches.

Find large files with Omni Disk Sweeper and check which app generates them.

https://www.omnigroup.com/more

Items buried in ~/Library might be difficult for an average user to spot and manage (iPhone/iPad backups, iOS/iPadOS updates, Mail attachments, 3rd party apps' data files, caches, iCloud Drive contents etc).

So inspect these folder sizes and then preferably use the related app (Mail etc) to decide if you really want to delete files (large Mail attachements etc) it has generated there. '~/Library/Application Support' might have lots of folders -- the related apps regenerate them if needed but then the apps are reverted to their defaults:

~/Library/Application Support/MobileSync/Backup

~/Library/iTunes/iPhone Software Updates

~/Library/iTunes/iPad Software Updates

~/Library/Mail

~/Library/Application Support

~/Library/Caches

~/Library/Mobile Documents

If I understand you correctly, you see large amounts of data written then at some point you see all that disappear?

If the is the case, it is possible that some app you use saves a large amount of cached data while operating. This generally gets deleted but with APFS and especially Time Machine, when the app deletes this data it may not immediately go away and appear as free disk space. It will get tagged as "available space for use" but is kept in the Time Machine case for 24 hours in case there is a restore needed.

As someone else stated, open Activity Monitor, select the Disk tab, click on the bytes written column to sort for bytes written and see what has been doing the most disk writes. These statistics are started fresh after a reboot, so if you have not rebooted for a long time, you may want to reboot then monitor any app listed doing large disk writes when you notice sudden jump is disk space usage.

Joe Darby wrote:

Hello

I don’t use Time Machine, but I tried that when I first got the mini and it failed for lack of drive space (even on external drive) and I turned it off and don’t use that external drive anymore

It sounds like Time Machine may still be compiling backups for that old destination drive on your internal storage drive.

Go to System Settings > General > Time Machine > select the old TM drive > click the - sign > click Forget Destination.

"M4 Mac mini internal drive space rapidly filling in real-time: [...]im not a programmer but I am somewhat savvy and understand the lingo and I’m not afraid of the terminal. TIA"

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Saving Space:

How Much is System Data taking up? It can increase quickly, and takes some effort to rid of it in-full. Removing the contents of ~/Library/Caches does rid of it, in-full. So, back up your Mac with Time Machine, and then perform my User Tip: How to Use Finder to Clear “System Data" on a Mac - User Tip

Edit: Removing the contents of ~/Library/Caches does not rid of System Data, in-full.