[mrtg] phisycal memory

[Andrea Bencini]

I want to monitor the use of phisycal memory on my windows server.
I have hrStorageUsed.7 and hrStorageAllocationUnits.7
To have the used memory I must multiply the hrStorageUsed.7 and 
hrStorageAllocationUnits.7 value.
How can I do this in my Target mrtg?

An other question.
If I put in my windows server an usb disk or an usb memory card the 
variables hrStorageUsed.7 and  hrStorageAllocationUnits.7 change in 
hrStorageUsed.8 and hrStorageAllocationUnits.8.
How can I resolv this?
Thanks
Andrea