How to Sync two external HDD

How to Sync my two external HDD (both are 2 TB) ? I have two external HDD for backup . One of them is connected most of the time with PC and other is used for only backup purpose , I want exact same copy of data in both drives . How can I sync them such a way if any changes in one drive automatically changes to other just like cloud sync . I have done lot of googling , but most of them are not active . I want a software which can detect any change in one drive and sync to the other or sync in time interval . Please suggest me .

Try Free File Sync which is open source.

I have tried FreeFileSync , but I have to open the app each time to sync , Can not make as a repetitive automatic process . I want automated app just like cloud sync apps if there is any change it should start syncing .