Some versions of linux have a strange issue upon boot, and a simple Google on "boot hanging udev" will show you that there are many possible reasons.
Let me add one more reason to the pile.
On some versions of Red Hat Enterprise Linux 4, if your system is configured to authenticate users to an LDAP directory of some sort, your issue could be that the system is trying to connect to LDAP before seeking its own hardware! By the time of RHEL 4 update 8 this issue was fixed.
The solution is to boot the system off the DVD to linux rescue mode, allow the system to mount the installation on /mnt/sysimage and to edit /mnt/sysimage/etc/nsswitch.conf and remove all references to LDAP in this file.
On the next reboot you should blow right by udev and watch your system start up.
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