DMA read/write errors while copying data, 5.4

[dave]

Hello,
    I was following procedure2 of the gmirror howto located at:
http://people.freebsd.org/~rse/mirror/
and had got to the part where i was copying the data from the first to the
second disk. I did:
dump -L -0 -f- /usr | (cd /mnt/usr; restore -r -v -f-)
which went for a while then hung. Checking the console i saw:

ad1: WARNING - WRITE_DMA interrupt was seen but timeout fired LBA=78165359
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567655
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567659
ad1: WARNING - WRITE_DMA interrupt was seen but timeout fired LBA=78165359
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567655
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567659
ad1: WARNING - WRITE_DMA interrupt was seen but timeout fired LBA=78165359
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567655
ad0: WARNING - READ_DMA interrupt was seen but timeout fired LBA=7567659

etc. I've done this on another box, a test machine and did not experience
this issue. Might one or more of these drives be going bad?
Thanks.
Dave.