Hard disk required for Windows system image, though it shouldn't be

Hello everybody.
I have 3 HDD, 500GB that I use for storing files, downloads and games - all the stuff I don't need much. Other is 1TB that holds 2 disks, 100GB system disk and remaining space for programs, music, documents - important stuff. And the third is 1TB and I want it to be used as a backup HDD. So what I want is to have a complete backup of system disk and important stuff and ignore the 500GB HDD. I set the backup to be performed everyday at 4 am since I don't turn off my PC. Recently I noticed that system image started to require that 500GB disk. I was wondering why? All it has is a folder with one game that I didn't even install - just copied, and a Downloads folder. I installed Dues Ex 3 on this disk but removed since. Can I somehow manually find the reson for this disk to be necessary for system image and eliminate it?

So what I need is to always have a most recent backup of one 1TB HDD on another 1TB HDD. And it would be nice to have a system saved as well so if my HDD crashes I won't have to reinstall everything. If this is too much to ask from Windows 7 I guess I'll just have the files backed up, without the system.

Hello Brink. That 500GB disk that I don't want to include in the backup indeed is listed as "System" in Disk Manager. I don't know how that happened, I installed Windows just a week ago, all my HDDs were formatted at that time. All I had done with that disk is install and removed Deus Ex, created "Downloads" folder and copied 2 folders with data from a USB drive. I've added "Downloads" to favorites though, will that count as a user folder?

No, the Downloads folder in User folders is different. I created a Downloads folder myself and added it to favorites. Here's the screenshot:


Disk 0 is for the backup, Disk 1 is the one I want to be backed up, Disk 2 is the one that is not supposed to be necessary for the system image.