1. It's better to do this on a separate hard disk. Imagine you are hit by some ransomware that encrypts your latest full backup. That should be kept off-line.
2. An external hard disk is fine. But, depending on the volume of data that changes in a month, a 64 GB USB-stick might sufficient (and use less power, and is easier to transport).
3. It has to be plugged in during the sync. It depends on how you do it, if that has to be always. One possibility is that you start it manually, each evening for example. Then it only needs to be plugged in that few minutes. Another option is that you run some program that syncs all changes automatically when they occur. Then it needs to be plugged in always.
At my home PC I have a batch file that copies new and changed data from selected folders to a USB-stick when I run it (a few times a week, maybe every day if there are many changes). It takes a minute or so to run. Since it's on the backside of a desktop that it's clumsy to reach, I leave it always in.
Another batch file that copies new and changed data from the USB-stick to another PC in my home network. Since I have to turn that on manually (and off again) I do that approximately once a week.
I store an external disk with images at my work. I make those every 2 months on the average.
I want to sync personal laptop data to another hard disk in case laptop HD failure. I already have an external HD to backup laptop data monthly in long interval. I understand there are cloud services like dropbox but don't like for privacy issues.
1) Should i buy another external HD to sync laptop data simultaneously?
2) Is external HD good option for file sync as it will be working long hours along with laptop?
3) Do i have to keep the file sync HD plugin to laptop always?
Looking forward your reply. Thanks.