It's certainly not normal. I have two USB external drives permanently connected with no problems.
I wonder if your AV is scanning the drive unnecessarily, or if Windows is frantically indexing it.
If your AV is set to perform a scan on startup, you could tell to skip this drive (as the only vector for a virus would be via you exiting drive, which is presumably protected already).
You could also turn off the indexing service for the external drive (My computer, right-click drive, properties, uncheck "Allow Indexing service to index this drive..."