Disconnect or unmount a usb drive using a script (Windows 7)

The computer is running Windows 7 pro in an office.  There are 2 usb drives connected to the computer. Let's call them D1 and D2.  A third party backup program writes to D1 on Monday, Wed and Friday.  It writes the backup to D2 on Tues, Thurs and Sunday.  The office staff must unplug drive D1 on Tuesday morning so they can take it home. D2 is plugged in on Tuesday morning.  
    Then on Wed morning the staff must unplug D2 and plug in D1.
Normally to properly disconnect  the usb drive someone has to manually click on the little uparrow in the task bar, find the correct icon, click on the icon and then click on the appropriate usb drive to dismount it.

The staff is not computer literate,  they just unplug one drive and insert the other.
What script or program could safely dismount the usb drive?

Thank you for your help.