5.3-RELEASE: WARNING - WRITE_DMA interrupt timout - what does it mean?

I just upgraded to 5.3-RELEASE a few days ago. This morning this line got 
into my system log file:
Nov  9 06:14:03 www kernel: ad0: WARNING - WRITE_DMA interrupt was seen but 
timeout fired LBA=2491143

I've never seen this message before. Can someone please explain what it 
means? With Thanks,

Zoltan

PS: I use the GENERIC kernel. My uname -a says:
FreeBSD www.xxxxxxxx.com 5.3-RELEASE FreeBSD 5.3-RELEASE #1: Sat Nov  6 
06:03:26 PST 2004     xxx_at_www.xxxxxxxx.com:/usr/obj/usr/src/sys/GENERIC 
i386