Fix for Unraid 7.3.2 hangs at "Loading /bzroot...ok" after upgrade from 6.12.15 — ASRock B250M Pro4 Mobo

Hello,

I just thought I'd share my experience of an Unraid 7.3.2 upgrade straight from 6.12.15 running on an ASRock B250M Pro4 motherboard. After the first reboot, the console stopped at Loading /bzroot...ok and would not boot any further.

tl;dr - I required a BIOS update from 1.40 to 2.50 to resolve the issue.

Following is the longer version of the symptoms I experienced, the troubleshooting and the final fix. This is the output from my Claude AI session.

Confirmed fix: Unraid 7.3.2 hangs at "Loading /bzroot...ok" after upgrade from 6.12.15 — ASRock B250M Pro4

Symptoms

• ASRock B250M Pro4, BIOS version 1.40, legacy/CSM boot (EFI- folder on flash)

• Unraid 6.12.15 had been running without issue

• Upgrade straight to 7.3.2 applied cleanly through the GUI, but the system would not boot afterwards

• With a monitor and keyboard connected, Loading /bzroot....ok was the last thing on screen, followed by a flashing cursor and nothing further

• No network presence — no response on the previous IP, no new DHCP lease, left overnight with no change

• USB flash drive LED stopped flashing at the bzroot prompt, indicating the kernel never got far enough to re-access the flash

Ruled out

• Flash corruption — bzroot loaded successfully, so the file was intact

• Plugins — Safe Mode behaved identically (expected, since the safe mode flag is read after the flash is mounted, which never happened)

• RAM — Memtest86+ passed five complete cycles with no errors

Fix
Flashed the BIOS from 1.40 to 2.50. No other BIOS settings were changed. Unraid 7.3.2 booted successfully and final configuration was completed in the new GUI.

Follow-up step required after the BIOS flash
Updating the BIOS resets all settings to defaults, which set the first boot option to UEFI: <Flash Drive Name>. Because the Unraid flash uses an EFI- folder (legacy boot), the UEFI entry will not boot it. The first successful boot only worked because the plain USB: entry was selected manually from the F11 boot menu.

Change the first boot option in the BIOS to USB: <Flash Drive Name> and save. Unraid then boots unattended, which matters for a headless server or after a power event.

Also re-verify after the flash: SATA mode set to AHCI, CSM enabled with Legacy storage and video OpROM policies, Secure Boot off, Fast Boot disabled.

Why it works
7.3.2 runs Linux kernel 6.18.38, against 6.1 in 6.12.15. BIOS 1.40 dates from around the board's 2017 release; the intervening revisions include CPU microcode and ACPI table updates that the newer kernel depends on. The hang occurred in early kernel init, before USB subsystem initialisation.

Worth checking if you hit this
Any similar-vintage board still on its original BIOS is a candidate. Check for firmware updates before troubleshooting kernel parameters or boot mode — it's a faster path than bisecting Unraid versions.