Wipe Free Space

[depth10]

After updating to the newest CCleaner 5.79.8704 on Windows 10, I Wipe Free Space anymore.  There used to be a checkbox when you clicked on Custom Clean and then all the way at the bottom.  But it's no longer there.  

So I looked up where to find and have found it under Tools then Drive Wiper.  So, I choose "Free Space Only" from the dropdown.  Then choose "Advanced Overwrite (3 passes)".  And then in the box where I can "check" which drives I want to select, my C:\ is grayed out.  I don't have any other logical drives mounted--only C:\

Why is it grayed out?

[Augeas]

Because your drive is an SSD, and WFS is pointless on an SSD. And WFS three passes on an SSD is three times more so.

[nukecad]

To explain a bit more:

CCleaner won't let you Wipe Free Space on an SSD because it's not needed and it can be detrimental to the life of the SSD.

WFS is used on a spinning disc HDD when you are giving/selling the drive (or the computer) to someone else.
It overwrites all the free space with random characters and then deletes them so that none of the files you have deleted can be recovered by the new owner.
If someone tries to recover files from it they'll only recover the random characters.
There is no reason for wiping the free space other than to prevent possible recovery of deleted files.

SDD's work differently and deleted files can't (normally) be recovered - so WFS is not needed on an SSD and would be a waste of time.
In fact it's a bit worse than that because running it frequently, especially with multiple overwrites, can shorten the life of the SSD.

PS. You should also find that you can't set secure deletion for an SSD either - again because overwrites are not needed and can be detrimental to an SSD.

[Andavari]

If wanting something "wiped" off the SSD just use Windows built-in Optimize Drives and it will run Trim on the SSD - assuming it's formatted with NTFS, otherwise it will do a regular defrag on it.