hard drive – How to make HDD a lazy copy of SSD?

I want to build a PC with 500 GB SSD and 500 GB HDD, and ask for other discs, but they are not important in this question. How to make that hard disk a silent copy of the SSD? I mean that all the data in SSD must be "reflected" in HDD in "lazy" mode. I have read about RAID, but as I understood, that is not exactly what I need, because in RAID SSD the writing speed will be limited by HDD.