System Data filled HD

Sure.

Let's start with the questions you asked initially.

What are the right questions to ask?

Looks like you already done a great job already with the questions you posted.

How do I learn how my HD filled with System Data?

Again, what "fills" System Data is controlled by macOS. The largest contributor is from Time Machine local snapshots. These are complete copies of your Mac's Macintosh HD - Data volume, and are created hourly. However, TM will remove them daily ... at least, it is supposed to. There are a number of methods to remove them "manually," but you shouldn't have to. One of them is to boot up your Mac in Safe Mode.

Ref: About Time Machine local snapshots - Apple Support

How do I see see what System Data has been stored on my HD?

Since TM local snapshots are the biggest contributor, you can get a list of them by using the following command in the Terminal app: sudo tmutil listlocalsnapshots /

How do I clear it safely?

In one of the following four ways:

1. Let macOS perform the work. (Recommended)
2. Boot up the Mac in Safe Mode.
3. Toggle off; wait 5-10 mins, then toggle on "Backup Automatically" from the Time Machine menu in System Preferences.
4. Use the tmutil command in the Terminal.

How do I manage System Data?

Again, it best not to micro-manage System Data, but just let macOS perform this task.

If there are any other questions you may have, related to this, please ask away.

"System Data" is controlled by macOS. It consists of things like: Time Machine local snapshots, log files, caches, etc.

One simple technique to reduce the amount of system data is to boot up your notebook in Safe Mode. Then boot it back up normally.

Ref: How to use safe mode on your Mac - Apple Support

alison.e.rose wrote:

1. Hi Tesserax,

re. your list of solutions in order of least risk, or preference:

How do I "let macOS perform the work"?

2. you previously sent a link to restart in safe mode. -- I can do this.

3. do you mean toggle off Time Machine, then wait, and follow the steps you suggest.

4. you said how to do this, but I'm hesitant to use Terminal. I will treat this as last resort

Yes, that would be in order of preference ... and to "let macOS perform this work" is simply don't micro-manage what is stored in System Data, but let macOS perform this task.

Documents is not part of System Data, and removing items from it would not affect the amount in System Data. The same would be true for all of the other categories that are not in System Data. However, removing these items should have an effect in the overall available storage on your Mac's drive.