HELP

When my USB disk (used for the backup of my (this) server) is connected to the USB port and powered on the Linux CentOS server will not boot after the SELinux: unregistering netfilter hooks message.

When the USB disk is turned off: the server is booting normally and all services are started and everyone can surf onto my sites again, send e-mail etc..

What can be the reason why Linux is not booting properly when my USB disk is on?