Time Machine Data Stuck in my HD

System Data used to be called "Other Category" in previous systems. The 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 Do

Antaggio wrote:

I forgot to connect my TM external drive for a while so macOS started taking snapshots and saving them on my HD. Now My HD is full, and even though there is plenty of room to save TM files on my external drive, Time Machine says it can't back-up because my HD is full. I have already tried to delete local snapshots and disable them by turning off TM automatic back-ups, but the huge data cluster is still sitting unchanged on my HD and does not budge no matter what I try. Will I have to re-format my HD?

"This may take a while. Plus, there’s also no interface to tell you if the local files are deleted.."

https://eshop.macsales.com/blog/77372-how-to-delete-time-machine-local-snapshots/

You can delete the local snapshots with Disk Utility:

Is this a related issue over here

Time Machine Data Stuck in my HD - Apple Community