What is this info mean?

[Jaga]

When WinRAR unpacks it uses a temp file on C:. When done it moves the file to the target. So it still is using my system disk. But yes, writes are halved.

Don't know - I haven't used WinRAR for over 10 years. I stick with 7zip now. The FTP software I use has a place where I can tell it what temp directory to use, so it never hits the system temp or tmp variable locations. 7zip also has a setting to override the system temp folder with your own. Perhaps it's time to re-evaluate using WinRAR? Check out 7zip and see if maybe it's a better solution for unpacking.

[RAMbo]

The usenet software uses WinRAR in the background. I have no control over it

The only thing that may work are symbolic links. But the last time I've tried they only work on the same drive.

[Jaga]

SymLinks will definitely work across different drives - you have to link the folder. If WinWAR creates it's own temp sub-folder, you could use a symbolic link to place it's destination on another drive easily.

The software I've used for creating symlinks in the past is called Junction Link Magic, though there are many out there.