adding disk moves ad0 to ad4

Matthew Seaman m.seaman at
Wed Feb 3 17:55:30 UTC 2010

Hash: SHA1

On 03/02/2010 17:35, Steve Franks wrote:
> Just curious, having read the handbook section talking about freebsd
> going straight to the hardware and skipping the bios for disk
> numbering, why then, if I stick a sata disk in 'sata0' on the
> motherboard, does it come up as ad0, but if I add a second disk in
> 'sata1' or 'pata0', on the next boot, I have no ad0, but ad4 and ad6?
> This seems to be the case with every mobo I've owned in the last 2
> years from a variety of mfr's.  Is there a way around this?  I don't
> care what it comes out as, as long as it stays put...  Since I have
> the only fbsd system at work, I tend to format alot of funky drives
> for people, and it gets anoying having to swap fstab's every time...

I've never seen disks spontaneously changing from ad0 to ad4.  I have
seen motherboards where toggling BIOS settings to do with PATA
compatibility / SATA support changes the device number.

Sata drives frequently appear as ad4 and ad6 -- that's because each SATA
connector is behaving like a separate IDE bus (with 2 devices -- master
and slave -- allocated per bus, even though you'ld only ever use one
with SATA disks).  Devices ad0 -- ad3 are allocated to the *real* IDE
busses on the motherboard -- probably one of those slots is taken up by
a CD or DVD drive.

Now, with some mobo's ad4 and ad6 is all you're going to get.  Others,
you can tweak some settings in the BIOS and your SATA drives will then
appear as ad0 and ad1.  Unfortunately it's pot luck as to what kit this
works with, and what it doesn't.



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