I got the same warning regarding firmware upgrade, I think it refers to upgrading your already installed 5,1 firmware. I needed to re-start 5-6 times before the firmware actually installed. Once finished my Early 2009 Mac Pro is now running High Sierra. A side note - I am using a Sonnet Technologies Tempo 6Gb/s SATA PCIe 2.0 Drive Card with dual Samsung SSD drives, ORICO USB3.0 4 Port PCI Express to USB3.0 Host Controller Adapter and a AUKEY USB-C PCI Express Card. All of which help keep me up to date.