> > Back in early December, I added two additional SATA drives,
> > filling up the nf4's 4 SATA ports.  Somehow the firmware got
> >
> > [...]
> >
> > In addition to this, NetBSD now gets the "lost interrupt" problem unless
> > I turn off APIC in the firmware.  (FreeBSD and Linux seem to be okay.)
> 
> Do you remember whether you were using both built-in nForce4 SATA
> controllers at the time where you only had two SATA drives? In my case, I
> have to disable APIC if I want to use both controllers.

Short answer: looks like the 1st 2 were on the same SATA controller.

Long answer:

FreeBSD called the 1st two ad4 and ad6, which are both on atapci1.
The additional disks are called ad8 and ad10, which are both on atapci2.
I assume that PATA hard drives would be called ad0-3.
FreeBSD seems to be reserving a spot for a second drive on each SATA
port, as if they were PATA channels.

atapci1: <nVidia nForce4 SATA150 controller> port 0x9f0-0x9f7,0xbf0-0xbf3,0x970-0x977,0xb70-0xb73,0xcc00-0xcc0f mem 0xfebfb00
0-0xfebfbfff irq 11 at device 7.0 on pci0
ata2: <ATA channel 0> on atapci1
ata3: <ATA channel 1> on atapci1
atapci2: <nVidia nForce4 SATA150 controller> port 0x9e0-0x9e7,0xbe0-0xbe3,0x960-0x967,0xb60-0xb63,0xb800-0xb80f mem 0xfebfa00
0-0xfebfafff irq 10 at device 8.0 on pci0
ata4: <ATA channel 0> on atapci2
ata5: <ATA channel 1> on atapci2
[ ... ]
ad4: 238475MB <Seagate ST3250823AS 3.03> at ata2-master SATA150
ad6: 238475MB <Seagate ST3250823AS 3.03> at ata3-master SATA150
ad8: 238475MB <Seagate ST3250823AS 3.03> at ata4-master SATA150
ad10: 238475MB <Seagate ST3250823AS 3.03> at ata5-master SATA150

Default NetBSD kernels of course just start numbering from 0, so
no helpful clues there.

Where in the BIOS did you find the controls to enable/disable the
individual SATA drives?