You may have read in another thread that I now have a NAS. The NAS supports NFS so it seemed completely natural to NFS mount a directory tree from the NAS on a desktop PC and to put this mount in the /etc/fstab file so it is mounted whenever the desktop PC is started. I did this and specified the 'bg' option so that if the file system concerned is not available the mount is retried in the background.
Upon the next reboot of the PC it got stuck in the boot process in such a way that all I could see was the Ubuntu logo and no text. Using the Ctrl-Alt combinations to switch virtual console did not make any difference (no text appeared) and Ctrl-Alt-Backspace to kill a running X server did not make any difference either. In the end I used the magic SysReq to reboot the PC.
Upon choosing the "recovery mode" from GRUB I found that the NFS mount was not working because the network card was not up. It seems that when you use the helpful GNOME-based network configuration to configure a network connection it then does not get started early enough in the boot process to support the NFS mount which is rather different than if it had been configured in the /etc/network/interfaces file.
At this point I also found that one RAID array was failing to start which, after some investigation turned out to be a zeroed-out RAID superblock on one of the partitions which I will discuss in another thread.
I now do not know if the failing NFS mount was responsible for the hung boot process and the superblock got corrupted by the magic SysReq reboot or if the superblock was corrupted by a failing drive and the NFS mount was a red herring but it was certainly somewhat surprising.
Steve.