MAC OS System Data using a lot of storage

What follows may appear generic but would apply you this situation

Manage Storage

The final word from Apple on Managing the " System Data “ Category

System Data: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins.

You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

There are areas of the Operating System which we users have control over and that is the User Account Folder ( Home Folder )

All other areas are not accessible 

It is generally a good computer practice to alway keep at least 20% to 25% of the Total Drive Capacity’s as Empty Space.

Allowing the computer to drop below these guidelines may eventually, cause unintended consequences.

There is Purgeable Space and there is Empty Space.

Purgeable Space which is Controlled by the Operating System. When the Operating Systems decides the computer needs additional Empty Space, it will move a portion of the Purgeable to Empty space

AFAIK - there is no User Actions to hasten this transition from Purgeable to Empty Space

It can day or longer before this will occur.

The links below will assist in identifying what is taking up space on the Internal Drive and provide possible ways to remove data that is under the direct control of the User ( Home Folder ) . 

Rebuild the Spotlight index on your Mac

What is “Other” storage on a Mac, and how can I clean it out?

Free up storage space on your Mac

GrandPerspective 

How to delete Time Machine snapshots on your Mac.  

Often caused if the Time Machine Drive has not been attached  to the computer and TM Backup is set to run on a Schedule. 

TM Backup will make Snap Shot on the Internal Drive awaiting the TM Backup Drive to be attached. 

Only then will the Snaps Shots be transferred to the External Drive and deleted the Internal Drive.

View APFS snapshots in Disk Utility on Mac - Apple Support (CA)

See used and available storage space on your Mac

Locate backups of your iPhone, iPad, and iPod touch

If there a Suite of Adobe Applications  used on this computer ?

They may create some very large cache files that can be removed. Though, the Adobe cache files will be recreated as the Applications needs them.

https://helpx.adobe.com/ca/premiere-pro/kb/clear-cache.html

Is System Data or Other Storage on Your Mac Taking Up a Lot of Space 

Files that are not included in the categories listed under storage are categorized under Other or System Data. This can change as the Mac determines where it’s categorizing those files, etc. 

I would recommend going into Safe Mode first since by doing this the Mac will delete system cache and if there is any erroneous calculations of data storage that hopefully will resolve it. 

You can check the sizes of all files and folders on your hard drive or just the library. 

To check size of all files in folders in just the library only:

1. Click on “Go” in the menu bar, hold down the Option key on your keyboard, and then select “Library” in the drop-down menu

2. Select List View in the toolbar of this window, so that everything in the window is stored in a list. 

3. Select “View” in the menu bar, and select “Show View Options”. Once that window pops up, then check the box next to “Calculate all sizes”. Close that little window, and then sort by size in the Finder window. 

4. You should be able to see what is taking up so much space. 

5. Also, you can select all the files/folders in this window, right click on the highlighted files then select “Get Info” to see the total amount of space that all files and folders in the Library are using. 

If what you see in the Library doesn’t add up to what the System Data or Other is taking up then check everything on your hard drive. 

To check everything on your hard drive:

Open a Finder window. Click on your hard drive under Locations in the left sidebar. 

If you don’t see your hard drive, click on Finder in the menu bar, select Settings or Preferences, click on Sidebar, then check the box next to the name of your hard drive. 

Once you’ve selected your hard drive in the left sidebar of Finder, select List View in the toolbar of this window, so that everything in the window is stored in a list. Select “View” in the menu bar, and select “Show View Options”. Once that window pops up, then check the box next to “Calculate all sizes”. Close that little window, and then sort by size in the Finder window. 

It may take a few minutes for the sizes of everything on your hard drive to calculate. You can click on a drop down next to one of the folders and see if any calculated sizes are starting to load. Once everything is loaded, you can see what is taking up so much space on your Mac.

If you see that the Container folder has a large file or folder contact the app developer that has a large container file for more information. 

If you’ve downloaded and then deleted Epic Games, you may see a lot of space still taken up by it under your Shared Folder. 

If you aren’t sure whether you can delete the data that is taking up the space in the Library or other folders, or need more information about it, contact Apple Support. 

I’m also more than happy to answer any questions. 

Also, after deleting any files, folders (that are safe to delete), and apps make sure to empty your Trash. 

There is a chance that copies of your photos are being made when you import them into your Macbook Air.

When importing photos into the Photos application, it could either be making a copy and storing into its library or it could be imported as a reference. (See "reference files" in (Change where your files are stored in Photos on Mac - Apple Support).

I would suggest figuring out which method of import Photos is doing and whether you need to remove the duplicates in your storage.

pdc4570 wrote:

Hi. I changed no settings and added no resources but the problem appears to have corrected itself.

It will be back. All your unzipping and moving around must have resulted in you deleting a bunch of files. On the Mac, deleting files tends to make you lose free disk space. 😄

You can't actually delete files. All you can do is flag them for deletion. Then, at some later time (of the operating system's choosing), it will run the "deleted" task to go through and delete local snapshots and actually free up those deleted files. In theory, that should happen automatically if you run out of storage. In practice, I've never seen it do that. It usually corrects itself after 1-2 days.

Since you have only 250 GB of total storage, this will be a regular experience for you.

In your screenshots, you are comparing two different things. One is your local storage. The other is your cloud storage. Since you have only 250 GB, you aren't going to be able to download all of your iCloud data. It looks like your iCloud settings are "optimized" so that most of your iCloud data is only available in the cloud and is not synced locally. That would explains the discrepancy.

In theory, it is possible to have a local copy of all of your cloud storage. But you would need a much bigger hard drive and you have to work at it. iCloud is quite finicky.

pdc4570 wrote:

my understanding is if I move the photos library to an external device then it won't replicate to iCloud and so my other devices won't have access.

Can you explain where you got that idea?

Apple officially documents and supports this: Move your Photos library to save space on your Mac - Apple Support

I don't see any indication that it won't continue to work with iCloud.

Seems Normal.

What were you expecting ?

A 256 GB Capacity Drive is not going to hold a lot of data especially Photos and Vids.

I would be wary if replying to a certain suggestion

You may not get a response or not get one you are expecting