Question Good free way to backup data onto a nas.

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Hello, I just got my NAS complete and hopefully will run for more than a day before going offline like last time.

Specs:
Intel Pentium N3530 Quad Core
8gb DDR3
HP S700 120GB SATA III SSD set as Windows Storage share because I couldn't figure out how to make it a cache drive for the HDD.
Seagate Barracuda 2tb SATAIII 7200RPM HDD set a backup share in Windows.
FreeNAS-11.1-U7 installed onto an ancient PNY 8gb SataII USB stick.

The 2TB drive is powered externally and plugged into the laptop's optical drive slimline connector using a modified SATA data cable, but it works. Kind of funny the bios says [ODD: Seagate 2TB]

Finally, as many times as I have heard USAFRet drill it into peoples heads, I want a solution to backup all of the files on the PCs in my house automatically.

As most of the PCs in my house have 1 or 2 small SSDs, the 2TB should be enough in theory. I'm not ready to buy another drive either.
What is a free and easy to use program that can do this?
 
Regarding nightly backups and time.

My C drive just ran its nightly Incremental to the NAS box, starting at 12:01.
3 mins 8 sec, 3.1GB.

That 3.1GB is the amount of data that changed between last night and tonight, on that drive.
The other drives will run at 0200, 0300, etc...