There have been issues with some chipsets.  I have problems with a
cd-rw drive with an alimagic chipset in an ASUS motherboard, but my
disks work fine.

pciide0 at pci0 dev 4 function 0: Acer Labs M5229 UDMA IDE Controller (rev. 0xc4)
pciide0: bus-master DMA support present
pciide0: primary channel configured to compatibility mode
wd0 at pciide0 channel 0 drive 0: <ST340016A>
wd0: drive supports 16-sector PIO transfers, LBA addressing
wd0: 38166 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 78165360 sectors
wd0: 32-bit data port
wd0: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)
wd1 at pciide0 channel 0 drive 1: <ST340016A>
wd1: drive supports 16-sector PIO transfers, LBA addressing
wd1: 38166 MB, 16383 cyl, 16 head, 63 sec, 512 bytes/sect x 78165360 sectors
wd1: 32-bit data port
wd1: drive supports PIO mode 4, DMA mode 2, Ultra-DMA mode 5 (Ultra/100)

Also, note that by adding another disk you are using more power.  I
would suggest checking your power supply with an oscilloscope while
the system has only the old disks and then with the 160 GB disk also.
Given that the disk being on a separate channel led to problems, I
would be suspicious of the power situation.

-- 
        Greg Troxel <gdt@ir.bbn.com>