Helping people with computers... one answer at a time.
An external hard drive does run certain risks when it comes to malware, but often not the ones we think of. Avoiding malware remains the best solution.
I have an external drive that I use to backup my data, and it is permanently connected to a USB port. Is there a significant risk that a virus could enter my system destroying all my data including the data on the external drive? (I use Norton protection package that comes with Comcast).
I won't call it significant, but yes, there is a risk.
And what you describe isn't even the biggest risk.
That being said, I'll put it this way: when coupled with good behaviour and good tools, I leave my external hard drives plugged in all the time as well.
I'll tell you why.
First, yes, it's quite possible that there are viruses out there that will simply seek out and destroy data on every device connected to your computer, including external hard drives and perhaps even remote network connections.
But those types of incredibly destructive viruses are pretty rare these days. I actually wouldn't expect to come across one that simply destroys everything.
Viruses are more ... purposeful ... these days.
They typically want to do two things:
send spam, or steal data and information
copy themselves to more systems
Destroying everything doesn't accomplish the first goal - in fact it's somewhat counterproductive.
It's the second goal we want to be wary of.
In recent years, several fairly major viral infections were the result of viruses that figured out how to copy themselves to USB thumbdrives. That way, when people inserted a thumbdrive into an infected system that drive became infected itself - a "carrier" of the infection. Then when placed into another uninfected machine the virus copied itself from the thumbdrive to the formerly clean system, infecting it.
Now, while I said "USB Thumbdrive", in fact many of the viruses would copy themselves to any removable media on your system.
Including - you guessed it - your external USB hard drive.
It is, after all, removable media.
So in my opinion you're actually more at risk that your external hard drive becomes infected as a carrier than you are for any data loss. If that drives become infected, and you then take it to another machine, you run the risk of carrying the infection with you.
For what it's worth, I also don't think leaving it plugged in all the time really has much of an impact one way or another. The act of plugging it in - perhaps to perform your backups - is enough for a virus to spread to the drive if your machine is infected.
I'm not meaning to be trite here, but the real solution is not to get infected in the first place.
Do all the things you already know how to do to keep your system clean. The net result is that as part of your system your external hard drive would be protected as well.
And I'm quite OK leaving it plugged in all the time. If nothing else it means that your backups are more likely to happen - be they manual or automatic. As you also know, backups are pretty darned important. Balanced against the practical risks we've just been discussing I'd much rather leave it plugged in all the time so that those backups actually happen.