If you have not fixed yet try disabling HCI in usb setting(BIOS). This solved the problem from another forum similar to this.
There is no option in the BIOS for HCI in USB options on the DL580 G5. The only BIOS options (at least in the latest 2015 version) for USB are to turn off external ports, all ports or disable 2.0. I have the same exact issue and have been troubleshooting. Disabling 2.0 does not fix. Turning off any of the ports does not fix. There are no BIOS settings that can help that I have located.
In his case the old "owner" may have created a bootable CD and installed to a previously created logical drive for the OS boot. The USB bus is having some issue with the MBR / format of the stick. My suggestion is to:
Boot from the FreeNAS install CD and when at the "Install/Upgrade" screen insert the USB stick at that time.
If the install is successful it probably won't boot because of the "boot_grub" label.
Boot the system off Gparted and change the label from "boot_grub" to "boot".
This should fix the problem. I cannot test yet as I have a hardware issue and need to solve that first. Once I have had a chance to test I will report back.
EDIT: Can confirm this method worked. The only thing is, I did not have to change the boot flag on the USB. It booted up correct, directly after the installation.