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The PS2 BIOS isn't upgradeable, although there may be ways to modify it once it has been loaded into RAM. All BIOS-es are loaded onto RAM on startup and if someone can get to the bare metal level and decipher and edit the BIOS once it has been placed in the RAM it may be possible to do anything with the system. Other than that, there is no way to modify the BIOS through software.
EDIT: about the hard drive, the Official Utility Disc checks the HDD firmware and then installs a Boot Sector to the HDD. The BIOS detects this Boot Sector and runs software from it (this (and because of a hardware detection routine) is why all HDDs spin up and read when you start your PS2). Currently, only Sony has the ability to create a proper boot sector.
cool, so now now we know ! Thanks for the info.
Two questions for the DMSers on the scene. (1) Do you have to turn on ATAD patching everytime you want a game to use the hard drive or see it in the browser or is it only necessary to turn it on to install the HDTools from the original Sony Utility disc. (2) If you are not turning ATAD patching on everytime but just during the initial install then is the ATAD patching not just allowing the PS2 to bypass the firmware check of the Hard Drive thus installing the tools and allowing the PS2 to read the drive for good once the tools are installed. The reason that I ask is because maybe there is a way to Hex Edit the original Sony Utility Disc to bypass this check and as long as you could load a backup you could load it and get the HD Tools installed and not have to have a DMS3 because to me it is a bit of a stretch to buy a DMS3 just to enable ATAD patching to install the HD Tools when we can boot backups and and load entire games off the Hard Drive already. I know that some things are not possible but it seems to me that it is only a matter of determining what the ATAD patching allows the PS2 to skip or ignore and modifying the disc to do the same. Do you all think that this is impossible or what??? Thanx in advance for any feedback.