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#21
4% is pretty small for a system drive shadow copy. That's what Winsxs is and allows for system restore points. That's not part of system reserved. System reserved area has it's own shadow file(s) to allow repair of the boot sector and some system settings.
You can increase the size of the shadow copy file by changing the system properties. You also can turn it off but you better have another method to restore your system unless you enjoy reloading the OS on every problem.
Added: You don't need a shadow copy on a backup disk. That what the whole disk is for.