Yes of course, my fault. It will downgrade after five of these.

Oct 15 20:55:48 antrax /netbsd: wd4a: DMA error reading fsbn 63916575 
(wd4 bn 63916575; cn 63409 tn 4 sn 51), retrying
Oct 15 20:55:48 antrax /netbsd: wd4: soft error (corrected)

>There should be errors messages around this as well (here we know it
>downgraded but we don't know why).
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Done that. Seems to help a little though.

>Yes, definitively.
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>>Secondly, is it possible to make the driver not downgrade the transfer 
>>mode when it experiences these DMA errors once in a while? They really 
>>only occur like once an hour or so, and that shouldn't be a problem.
>>    
>>
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Ah, I knew about that option, but didn't know it would stop the driver 
from downgrading from there. By forcing the drives to UDMA66 they no 
longer get those DMA errors at all, so I guess I'll have to be happy 
with that. Thanks.

>Yes, with flags on the devices in the kenrel config file. See wd(4)
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--
Thomas Hertz