Thank you JPWhite for your input: nice that you tried noapic. I can confirm your observations, but it looks like we need only the BIOS 'combi' setting.

A few tested options tried for my DELL 8400, with 1 SATA disk.:
BIOS "bootsetting" initstatus, SATA disks/ remarks
autoraid/ahci "knoppix noapic", OK, not found
autoraid/ahci "knoppix26 noapic", stalls, ata_piix abnormal status port0x177
autoraid/ata " knoppix noapic", OK, not found, cpu0 & cpu1
autoraid/ata "knoppix26 noapic", stalls, SATA devices observed
combi "knoppix26 <none>", stalls, sda1 & sda2 observed
combi "knoppix26 noapic", stalls, sda1 & sda2 observed
combi "knoppix26 acpi=off", OK, found, partitions sda1 & sda2 mounted
combi "knoppix <none>", OK, found, partitions hda1 & hda2 mounted
combi "knoppix noapic", OK, found, partitions hda1 & hda2 mounted
combi "knoppix acpi=off", OK, found, partitions hda1 & hda2 mounted

The simplest setup seems: (1) BIOS "combination mode' & (2) boot: knoppix <none>.
Tested kernel: knoppix 3.7 (kernel 2.4.27 and 2.6.9)

Added remarks:
- with two SATA disks in a DELL8400, with BIOS ''combi" mode only the first disk will be found.
- set the drives in the BIOS to "RAID autodetect/ATA" and both disks will be detected.