Hi Hexnut,
The drive label "c:\" normally means the drive is being monitored using SNMP or WMI whereas "C$" means the drive is being monitored via RPC.
Having that said, you could create the Drive Space monitors in two sweeps but that is overcomplicating things for nothing. I would simply go into the Device, select all the drives and click the option to Clone them. You can then rename the clones to whatever you like and set the Free Space threshold lower.
Hopefully this makes things a little easier.
Sincerely,
Chris Foley