Nassvc.exe is the executable running for the NAS PM Service, which sends those WOL packets to the NAS device for the Auto power feature.
Without the service running, the Tera/Linkstation needs to be set to On or Manual, otherwise it will power down.
To disable it, go to Start->Run, type services.msc, hit OK. Find the NAS PM Service, right click, Properties. Set the Startup Type to Disabled, click the Stop button, and click OK.
Everyone with the CPU usage issue, try restarting the service, and updating the NASNavigator software to the latest version. It doesn't peg my CPU, and my computer's pretty darn old. Any relevant Event Log entries would be helpful too.