I have a Precision 7920 Tower, BIOS was up to date and everything working fine, but in my infinite wisdom I decided to install a non Windows OS, boot usb wasn't detected, so within the BIOS I enabled support for legacy OS's, then restarted and now I get only the blinking white cursor on boot, and neither F2 or F12 makes any difference.

I've tried removing the CMOS battery and moving the CMOS jumper from PSWD to CMOS but that doesn't make any difference. Chatgpt tells me that downloading the BIOS exe and renaming to BIOS_IMG.rcv and putting on a FAT32 USB can help update the ROM, but I've tried that multiple different sized USB sticks (250 Mb, 2GB, 32GB), but it never progresses to even try and update the ROM, so I don't know if this is the correct approach or not. I see for other devices using the /writeromfile or /writehdrfile parameter is used to create the rcv file, but it doesn't work for the bios on the 7920. 

I'm not using a 4K monitor, its a very old monitor with a DP (7920) to VGA (Monitor) cable. Its worked fine with the device up to now, so I'm assuming there's no issue there. 

So.. my question is (1) what is the correct way to create the BIOS_IMG.rcv for the 7920? Anything I need to be aware of in terms of usb drive size or usb port to connect it to? Any other issues you can think of that have prevented BIOS upgrades happening before? Is there any known issue enabling Legacy OS support on a 7920?