It's safe to keep an external hard disk always connected to the pc ?

I synchronize manually. Data that is really important gets backed up pretty much the moment it is created. Lower priority data gets done around once a month. I never have backup drives connected when not in use. Apart from the possibility of being compromised if the system is, there is the wear and tear on the drive. I've never lost one of my cold storage drives but I've had many system drives that have been on all the time die on me.