'Other' storage

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

Other / 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.

Congratulations on being willing to research this issue. That's how we learn. Yes, the "Other" does contain a lot of data. The following may be helpful in understanding what it contains.

The Other Category should not be altered, modified, move or deleted since it is part of the Mac’s operating system. Doing so would corrupt the OS and render the OS inoperative requiring a Wipe and Re-Installation.

The Other category is a potpourri of files which 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

• Fonts, plugins, extensions

• 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

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.

This may be a good time to seriously consider adding an external drive for storage. (Do not use your Time Machine disk.) Then you can move files onto it and then delete them from your HD to clear needed space. Easy to do and problem solved.

This computer maybe experiencing the of Numerous Accumulated Effects of Several Software UpGrades in the life of this computer.

Predicated on that as being true, the only ways to get around this

1 - Follow the advise already presented

2 - Getting a Larger Capacity Computer

3 - Doing a drastic Scorched Earth Method 

Scorched Earth Method

This method will WIPE ALL Data and there is No Recovery - Period.

To perform this action will require booting from a Bootable Installer.

The Bootable Installer can Only be performed on an Apple Computer

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can not be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Once that is done read on for preparing the Destination computer >> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer. About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive.

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above.

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS.

7 - Follow the prompts and it may automatically reboot several time.

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

If the " Scorched Earth Method "   had been done as per above instructions - there would Only the Operating System installed and nothing more.

Now, if the the user ( you ) Migrated Everything that was done on the Time Machine Backup from when the existing issues existed, that is a different matter.

Issues that may exist on the Older Installation will be migrated to the New and Pristine Installation.

Thusly re-intoduced the pre-exiting issues right back into the computer.

Please allow me to clarify what you said for better understanding by the poster concerning the "Other". As noted above, there are numerous files such as fonts (which are extensive) which are installed as part of the computer's operating system but which are then listed under the Other category. Seven with a clean newly installed OS there will always be some Other items listed. As you noted, installing other items can certainly increase the Other.

As this is intended and directed to yours truly - yes I agree on Fonts taking up Space.

Though, in the context of the original statement by OP " I have a 1Tb HD of which 850Gb is used. Over 400GB is 'Other'. ". I tend to think Fonts may be part of the figure but not enough to warrant the issue the OP infers.