Suddenly 400 GB of free space on C drive

I have a 1TB C drive (boot drive) that has been gradually filling up until I had less than 40 GB free. Today I checked the properties of C and saw that it suddenly has 444GB of free space!

I had been using WinDirStat periodically to see what was taking up the space. Until today, WinDirStat showed something called "Local Disk" with an empty extension (that is, just "." with nothing after the dot). LocalDisk was taking up 381.5 GB and was the largest single contributor to disk usage.

Today when I ran WinDirStat, it showed "Local Disk" taking only 7.0 GB.

All of my user data seems to be intact. WinDirStat shows the same sizes for photos, music files, etc. as it did before today.

Nothing special happened between yesterday and today. I did not reboot the system. The system did not crash. There were no Windows updates. I did not run any cleanup utilities or do any other kind of system maintenance. I didn't even "empty the trash." CrystalDiskInfo indicates that my C drive is in good health.

How do you explain the sudden appearance of 400 GB free space?

Do you use System Restore? How much space do you allot to it?

What anti-virus, anti-malware, and security programs do you use?

Do you use any backup applications, including "cloud" backup?

I have never used system restore. I checked System Properties -> System Protection and clicked "Configure" button. The "System Protection for Local Disk (C:)" window shows Current usage is 1.13 GB and the Max Usage slider is 465.71 GB (50%). 50% must be a default because I never configured System Protection.

Do you think the system deleted an old restore point because the it saw the disk getting full?

In answer to your other questions: I run Malware Bytes (premium edition) and Avast Antivirus (free edition).

I have a Network Attached Storage drive, and I do regular backups. The last backup was a few days ago.

I should have mentioned that I am running Windows 7 64-bit "home premium."

I have not ever run system restore. I checked System -> Properties -> System Protection -> Configure and see that Current Usage is 1.13 GB and Max Usage is 465.7 GB (slider is at 50%). That must be a default because I never configured system protection (I didn't even know about it until now).

Do you think the system saw my disk space getting low and automatically deleted an old restore point?

I run MalwareBytes premium edition and Avast Antivirus free edition.

I backup my user files regularly to a Network Attached Storage device.

There are a number of thread on this forum about a defect in Avast that causes the space allotted to System Restore to grow uncontrollably. It is also discussed on the Avast forums.

The fact that the slider is set that high leads me to suspect Avast.

Avast is aware of the issue and may not fix it for a month or more.

People usually complain about losing space, not gaining space, and Avast is often the cause.

I'd guess that your "gradually" filling up as you said is due to the Avast moving that slider up, particularly if you have no other explanation as to why your drive was losing free space.

Thanks for the explanation. I found the thread you mentioned in the Avast forum. I will pursue a solution there.

If Avast is causing the problem, it will do so no longer! I replaced it with Panda.

I use a different AV on different OS installs ... I like it that way ... if an OS/AV doesn't work, i can still boot up ...

Here is a link to post #9 of what i know about fixing the problem with Avast
the dwindling free space on my C

I do use Avast or Panda or MSE or ..., they all work for me.
I make no recommendation of the best AV and security setup for anyone else ...

I'm running 6 real-time security programs on my ancient triple boot 2core Athlon II test box, and it works for me ...