Can FreeBSD safely use a (un-booted from) drive that is invisible to the BIOS?

[Danny MacMillan]

Hi,

I have a machine with the following two drives (as listed in dmesg):

ad0: 12427MB <Maxtor 91303D6 GAS54A12> at ata0-master UDMA33
ad2: 76319MB <Maxtor 6L080P0 BAJ41G10> at ata1-master UDMA33

ad0 is the boot drive.  It is recognized by the BIOS, obviously, and
has been in the machine for some years.  ad2 is a new drive I just
added to the machine yesterday.  It is not visible to the BIOS at all.
If anyone can posit a reason it would not be visible to the BIOS, I
would like to know the answer.  The BIOS supports LBA and ad0 is more
than 8GB so it wouldn't appear to be the 8GB limit, and the next limit
I am aware of is comfortably larger than 76GB.

At any rate ... it is not visible to the BIOS, but it is visible to
FreeBSD.  Since I'm not booting from the drive, I think it shouldn't
matter ... but when I use Fdisk from sysinstall I get the following
familiar error message:

|WARNING:  A geometry of 155061/16/63 for ad2 is incorrect.  Using  ¦
¦a more likely geometry.  If this geometry is incorrect or you      ¦
¦are unsure as to whether or not it's correct, please consult       ¦
¦the Hardware Guide in the Documentation submenu or use the         ¦
¦(G)eometry command to change it now.                               ¦
¦                                                                   ¦
¦Remember: you need to enter whatever your BIOS thinks the          ¦
¦geometry is!  For IDE, it's what you were told in the BIOS         ¦
¦setup. For SCSI, it's the translation mode your controller is      ¦
¦using.  Do NOT use a ``physical geometry''.                        |

Since I don't actually know what the BIOS thinks the geometry is,
I got cold feet and decided to ask the list.  I don't =think= it
should matter, since the BIOS shouldn't ever touch the disk, at least
as far as my understanding goes.

I do have one concern.  This drive was purchased more or less to act
as an emergency backup of the drive that's already in there.  If ad0
ever fails, ad2 drive will have to be put in a new machine whose BIOS
recognizes it in order to boot.  If I accept the mystery geometry for
the drive today, will I later face a problem where the BIOS disagrees
and the drive will be unbootable?

Thank you for your kind attention.

-- 
Danny MacMillan