Norton Antivirus for mac running for three days straight

I have an external drive, which has 540.56 GB used, and 459.3 GB available.  It's been scanned by Norton Antivirus for 3 days straight now, and it currently says:

"Scanning HD...

7677905 of 759272"

 

Or, in other words, it's scanned over 10 times the amount that I have.

 

Any idea what's going on here or how to stop it?

 

Thanks very much.