What is the SYSTEM group in Windows 7?
Solution 1:
I would probably leave it be. As I understand it, SYSTEM is used by many services etc. Actual users cannot put themselves into the SYSTEM group without being an administrator (and even then, I think you've got to do it with some strange method..).
Solution 2:
The NT AUTHORITY\SYSTEM account is used by most Windows services (via LocalSystem). Removing it won't prevent the Windows kernel from accessing the drive, but it might break some services that do it.