It isn't a disk driver since the OS is not loaded yet. It is the hardware identifying each disk in the SCSI chain. Not sure what else it was doing walking the bus much I know finding all the disk was the longest part.
I like how posting got fairly fast. Then we started putting absurd amounts of ram into servers so now they're back to slow.
Like we have a high clock speed dual 32 core AMD server with 1TB of ram that takes at least 5 minutes to do it's RAM check. So every time you need to reboot you're just sitting there twiddling your thumbs waiting anxiously.