Judging by the responses you have so far, 2 things come to mind.
1 Your question is not clear in what you are trying to do, "he wants to clone the data hard drive to keep a backup." but then you talk about Sync toy, which isn't really a clone or a backup tool, so do you want a sync, a clone or just an automatic backup of the data drive?
2. There are lots of options and everyone seems to have their favorite
"If you just want a file sync, try freefilesync or SyncBack SE"
"Paragon has some programs for this but there are many solutions out there."
"Cobian backup"
"Syncing is not the same as cloning. Not even remotely. The closest you can get with the two features is something like Windows Storage Server/Home Server."
"is this for the purposes of keeping a bootable copy as a back up? If not just get something like teracopy.....Other wise Acronis has a decent cloning tool"
"It is not clear the data protection level you are seeking but if it is for HDD failure then RAID1.... If it is for protection from local disaster then CrashPlan"
"Partition Magic has a very good disk clone wizard."
"Macrium free will do this"