Any free backup software for external hard drive ?

AAC

Recently I set up one external hard drive (DIY hard drive + enclosure) to backup folders and data from my desktop daily.  
I'm manually using copy & paste to copy all folders and files from desktop to external hard drives.  It takes long long time to complete the whole process (likely tons of old picture are repeating backup each time) but in fact there are less than 20 files are being modified or created everyday.   I'm looking for free and smart backup up software to back up only revised or newly created files in a day.  Any suggestion for me ?  Thanks.