iMac storage full, by "Other Users"

Recommended by who?

Paolore wrote:

I even bought and downloaded the 'recommended' Disk Clean Pro,

Recommended by who? FWIW there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

If you need cull out files to free up space on your drive consider the following:

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. 

Paolore wrote:

and I can't enter to check and delete.

What do you mean by this? You cannot boot to the internal macOS boot drive?

If you completely ran out of Free storage space on an APFS volume, then it can prevent you from deleting anything to free up space due to how the APFS file system works.....it needs a bit of space to apply modifications before actually removing the item....makes the file system more stable if the process is interrupted due to a system crash.

There is a possibility enough data may be stored within a hidden APFS snapshot that you can try to delete the snapshot. You can use the information in this Apple article to view & delete APFS snapshots:

View APFS snapshots in Disk Utility on Mac - Apple Support

Definitely agree with @Old Toad that you should avoid those types of apps since they usually cause more problems than they solve.

FYI, you should always have at least 20GB+ of Free storage space available at all times for the normal operation of macOS. With some workloads (like video editing) you should always have lots more Free space. Unfortunately most places within macOS do not actually show the Free space. The only place macOS actually lists the Free space is within Disk Utility when you click on the "Data" volume....on the right pane just beneath the storage bar graph is noted the "Free" space.

Everywhere else within macOS it instead shows "Available" space which unfortunately is very misleading. "Available" space is not always ready to be used at this time, since it is composed of "Purgeable" space which will be released for use at some unknown time in the future + Free space can be used immediately. Purgeable space is usually listed in parentheses after the "Available" space.

Free space =  Available space  -  Purgeable space

"Other Users" is a macOS designation found only within the Storage Management area and possibly Disk Utility. No one else, especially third party apps will see any area designated as "Other Users".

Historically, "Other Users" should only refer to other macOS user accounts on the system. Do you have any other user accounts for login? You can check the "Users & Groups" section of System Preferences. If you have other macOS user accounts, then log into them to look for large items. The only way Grand Perspective & other similar apps can see inside those other user accounts is if you are prompted for an admin password when using the app since it requires admin access to see those areas.

Unfortunately I have seen reports on these forums which mentioned the "Other Users" may refer to items both within the hidden user Library, or even items related to system use.

Also, the values associated with these storage designations is known to be inaccurate.

Did you look for APFS snapshots like I mentioned previously? I'm not sure macOS 10.13 High Sierra's Disk Utility has an option on the "View" menu for viewing APFS snapshots (I don't have access to a system that old at the moment to check). If not, then I can provide instructions for another method for viewing & deleting APFS snapshots.

You should uninstall Disk Cleaner Pro according to the developer's instructions.

Some of the Other Users may be local snapshots made by the system. You can go into Disk Utility and look for the local snapshots. You can erase them (I'd leave the latest one):

How often do you reboot? If you leave the Mac on all the time and just sleep it I would recommend rebooting at least 2-3 times per week. This clears out temporary system and application cache and swap files.

And move enough files to the external drives to maintain a minimum of 80 GB of free space at all time. This will definitely help the performance. I keep my music library and my very large Photos library on an external drive. I have other Photos libraries on my boot drive for testing, etc.

Also the graphical storage representation is not as accurate and selecting the item in the Finder and bringing up the Info pane for the size of that item.

You have to click on the container disk before the local snapshots will be visible.