Just spent countless hours with this issue on a Supermicro X9Dai motherboard. After playing around with the Clover EFI bootloader, all of a sudden the motherboard got stuck on POST, with the B2 status code.
I could not get into either the BIOS nor the "Select boot device" screen. This made it impossible to flash a new bios or clear the NVRAM, which seems to be the main issue in my case.
After a bit of Googling, I came across the SuperMicro UEFI BIOS Recovery guide and it worked! This basically lets you boot into a BIOS recovery mode using a USB dongle and a latest BIOS file. Here's the steps:
- Using your laptop or spare pc, create a bootable FreeDOS USB dongle
- Download the latest bios from your motherboard's website. In my case (X9Dai) there is a link "Update Your BIOS" on the motherboard's page.
- Create a folder in the USB dongle and extract the BIOS files to the folder. Name the folder something easy like "BIOS" so you can "cd" into it later.
- Leaving all the files in the new folder, make a copy of the BIOS file to the root of the USB dongle (/) and rename it to "Super.ROM" (case sensitive!). The bios file is the largest (about 16mb), named something like "X9DAi5.XXX".
- Now insert the dongle in the X9Dai motherboard, and as soon as the keyboard powers up, hit CTRL+Home until it does two short beeps. It's a distinctive beep, so you'll know. Then wait and see if the BIOS comes in to "APTIO Recovery mode". I had to do this a couple of times to actually get in, not sure why. From here you can set "Reset NVRAM" enabled and "Proceed with Flash Update". There is also an option to "Reset Main Blocks" (something similar), I also enabled that.
- When done, you'll reboot and should be able to boot again to the FreeDOS USB!
- (MAY BE OPTIONAL) When booted on the FreeDOS USB, I "cd" into the BIOS folder and re-flashed the BIOS using the "AMI.BAT BIOSFileName.###" command. I'm not sure if this helped, but my BIOS was out of date anyway so I did it.
Important: When flashing the BIOS WAIT UNTIL THE DOS PROMPT SHOWS UP AGAIN, ONLY THEN IT'S COMPLETE. After flashing the BIOS, I powered the PC off, disconnected power, removed motherboard battery, cleared CMOS, and put everything back together. Then booted it up, got into bios, Loaded Defaults, saved, and reboot.
Hopefully this will save someone not going through hell like I have.