> Mar 28 23:52:17 zippo.unna.ping.de /netbsd: ahc0 at pci0 dev 12 function 0ahc0: 294x Single Channel, SCSI Id=7, aic7870, 16 SCBs
> This one times out on my ccd every so often.  

I think you should check SCSI terminator setting and select short 
thick SCSI cable. Wrong terminator setting or bad cable cause
time out frequently.
Though...

> Even worse, the machine will 
> not come back to normal operation.  Even more worse, it handles write access 
> to my HP DAT much worse, i.e. times out and halts the system, than the ncr 
> in my old board (which burnt down last week due to fan failure).

In my limited experience, NetBSD stock aic7xxx driver never recovers 
from SCSI time out (system hangs, or panics). Pete's new one (based 
on Jan version of FreeBSD's) is better (it never panic or hangst), 
and latest one (based on Mar version of FreeBSD's) correctly recovers 
from time out.
But latest one seems to be more sensitive to SCSI cable problem,
than stock one. (But, I could be wrong...)
--
soda@sra.co.jp		Software Research Associates, Inc., Japan
(Noriyuki Soda)		   software tools and technology group