You can set the program monitor to 1/4 resolution, but that will not really help with a 6 camera shoot if you are serious about the singular 'my hard drive'. That is not sufficient. Get some more disks and possibly raid them.
I'm running media off an external thunderbolt drive that has two spindles and indeed RAIDs them. It does 4 cameras ok, but bogs down on the fifth.
Unless you're working with R3D or Alexa clips, this is more likely a limitation of your CPU than your hard drive.