What do these warning messages mean (READ_DMA UDMA ICRC error)?

[bob self]

I rearanged my hard drives / dvd burner and am now seeing these 
messages. The system SEEMS to run ok though.

ad0: WARNING - READ_DMA UDMA ICRC error (retrying request) LBA=119267359
ad0: WARNING - READ_DMA UDMA ICRC error (retrying request) LBA=119267359
ad0: FAILURE - READ_DMA status=51<READY,DSC,ERROR> 
error=84<ICRC,ABORTED> LBA=119267359
g_vfs_done():ad0s1f[READ(offset=44958728192, length=45056)]error = 5
vnode_pager_getpages: I/O read error
vm_fault: pager read error, pid 808 (firefox-bin)
ad0: WARNING - READ_DMA UDMA ICRC error (retrying request) LBA=119267359
ad0: WARNING - READ_DMA UDMA ICRC error (retrying request) LBA=119267359
ad0: WARNING - READ_DMA UDMA ICRC error (retrying request) LBA=119267519

thanks,
Bob