already active DMA on this device FAILURE at startup

[s m]

hello all,

i have FreeBSD8.2 on my system. occasionally,  i see some strange errors in
startup and freebsd can not boot correctly. when i reset my system
everything is ok and freebsd boot successfully. these are errors which
appear in statrtup:

ata1: setting up DMA failed
ad3: WARNING - READ_DMA requeued due to channel reset LBA=25167888
ata1: FAILURE - already active DMA on this device

these errors repeat more and more.

i don't know what's wrong in my freebsd. i have used freebsd for several
years and have not seen these errors before.
last few days ago, i set a separate partition for etc. can separate etc
partition cause this problem? how can i solve it to never see these errors
again? (i need a separate partition for etc)

thanks in advance,
SAM