Dear tgbennet,
By default, the eligible disks are set to the following order:
1 Locally attached storage (local)
2 Network storage (remote)
3 USB disks (usb)
So if you say "install --firstdiskdisk' , it will obviously install on the local disk. So suggestion would be to change that to "--firstdisk==usb".
Please refer the documentation center install/upgrade script commands for more details.
~dGeorgey