Thrashing external HDD after delay??

Hi all...

I'm running Win7 64-bit on a new Asus laptop. I have multiple external USB 2.0 HDDs connected to this laptop via a powered USB hub, one a 2TB Fantom Greendrive and a second a 1TB archive drive. The 2TB drive is my multimedia drive, and contains most of my MP3 files, graphics files and other non-application and backup stuff. I use the 1TB drive as a secondary backup. Both are left connected to the hub at all times. This PC is hooked to a keyboard/monitor which is shared with a second computer (a Macbook) via a KVM switch.

What's happening is that after about 28 minutes of inactivity, the Fantom HDD begins to rapidly click as something begins thrashing the drive. This usually happens while I'm working on the Macbook, so the Asus is receiving no input form my keyboard/mouse (the KVM switch has switched to the Macbook). If I click back to the Asus laptop via KVM switch, the clicking immediately stops.

I've checked the system monitor and I don't see any corresponding spikes in memory usage for the time immediately preceding the switch back to the Asus laptop via the KVM switch, nor has any of my multiple antivirus or malware scans (AVG, Avira and MalwareByte's Anti-Malware) detected any infection. I also checked to make sure that the thrashing is not the result of any automated antivirus scans (all are set to on-demand scanning) or automated backups (disabled - I back all files up manually).

Last, thinking this might be caused by something lurking in my screensaver, I disabled all screensavers and the behavior continued. It seems to happen a few minutes after the when the "Turn Off Display" from power management is invoked - I have power management set to turn off my display at 20 minutes and the thrashing seems to begin about 8 minutes after that (??). This seems like something that would be the hallmark of malware ("start activities only when the monitor is switched off so the user can't see what's happening"), but all my scans come up clean.

Any ideas?? I'm not so much worried that some piece of malware is scanning me (although I still think that's possible), as I don't hear the main C: drive being thrashed, nor is the other 1TB drive being thrashed - I'm more concerned about the drive getting trashed/worn out over time due to all the activity.

Help!!!

Nobody?