iMac System Data High?

System data is storage used by the operating system for functions you have no control over. It includes many things. One of them is snapshots.

Open Disk Utility and highlight 'Macintosh HD volumes' on the left sidebar. Then click the View pulldown menu and click Show APFS Snapshots. If you have Time Machine enabled you will see a bunch of com.apple.TimeMachine snapshots from today and yesterday. When attaching a Time Machine backup drive, these snapshots will be copied over to the Time Machine drive and cleared from the internal disk within 24-48 hours. If you haven't been attaching the Time Machine drive, you may have more than a couple days worth of Time Machine snapshots piling up and taking space.

You might also have some snapshots created with something other than Time Machine, their names will be entirely different in that case. So look for any old ones that are weird. You can optionally delete those but do not delete the Time Machine snapshots. Instead, get them to clear by connecting the Time Machine drive and waiting a day or two.

The latest version of DaisyDisk in the App Store added additional support to identify System Storage items and to help you clear them if necessary. It's a great tool to manage disk space as it can help you find large files taking a lot of disk space.

Brett, start an online chat session with Apple about this. I did this earlier today, and within 5 minutes was set up with a senior tech support guy who spent an hour and a half with me on line, holding my hand through a large number of (to me) complicated steps that I do not really understand, with the net result that my System Data has dropped from 400 to 41 GB, so I am very happy. He said anyone with this problem should reach out to Apple as there is no app, utility, or other ready-made answer, as every case will be individual. We still don't know quite what caused my System Data to get so huge, but at least (for the time being) the problem is solved. By the way, I do not have Apple Care and my 2021 MacBook Pro is of course long out of warranty

The System Data , previously called Other category, is a potpourri of files which can include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

• Files created and modified by other user accounts on your Mac.

They can be located anywhere on your hard drive.

The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use this free app, GrandPerspective, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Some users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

• Daisy Disk Hidden Space (new in ver. 4.25 Apr 16, 2023) significantly improved discovery of hidden space by including more system areas into the scan. In many cases nearly all hidden space can be revealed.

https://daisydiskapp.com/guide/hidden-space

DaisyDisk does nothing to shrink System Data

Maybe, but it did nothing for me yesterday.