I've been using Fc4 for a couple of years. Two months ago I tarred up the system and moved it to a new box (identical hardware). I ran kudzu just in case, and everything ran fine for 1.5 months.

Today the system hung with the DISK light on, I could only hard shutdown/reset. How can I determine the cause of the hangup?

It looks like the dmesg log is deleted upon each reboot so there was nothing for me to find. Any ideas?

I would start the search by readin /var/log/syslog and /var/log/messages if they contain information older than last reboot; also if there are other interesting logfiles in /var/log check them out; that's a start at least..

As a second alternative you could try to reproduce the problem, by doing exactly what you were doing then, in case it is a bug and not just a coincidence, and then try to think what exactly could be the reason. The log file search would be more meaningful though, since reproducing a thing like that can be quite a big job.