IMac start-up disk suddenly becomes full

Question

Hello!
I've got iMac 27 model id: iMac13,2 with 1TB disk with all latest software updates installed. All my files and folders normally count up to 300 GB, so around of 700 GB are free. But some time ago, I saw that my disk is full, 999,2 GB is used. DaisyDisk shows that I have 999,2 GB used, of which files and folder are only 250 or so GB and what is the rest is unknown.
Info in 'About this Mac' -> Storage shows that there is something marked as 'Other' that takes around 750 GB. After few hours the space frees up by itself. And this may repeat 3 or 4 times a week.
The most popular solution on the Internet for the problem when the disk comes full by itself is to switch off Time Machine, that creates local backups while external storage is unavailable. But Time Machine at this Mac is totally switched off so it can't be a reason.
Yesterday it was again 999,2 GB full, but now 700 GB is free. I have run a Hardware test and everything is OK.
What can be a problem? The only thing I suspect is disk malfunction.

Answer 1

To remove files from the 'Other' section you may try resetting your Mac to factory settings.
First of all, back up your system.
Restart your Mac. Once it restarts and the grey screen appears, press and hold the Command and R keys.
Select Disk Utility and click Continue.
Select your startup disk on the left and click the Erase tab.
Choose Mac OS Extended (Journaled) from the Format menu, enter a name, and then click Erase.
Select Reinstall OS X, click Continue, and then follow the instructions on your screen.