Update Think this may be because I have some problem booting off lvm - still in need of any help anyone could offer - even just a pointer or two as to where to look would be greatly appreciated.
One of my 11.10 machines will not boot - and I am at a loss as to why and what to do about it: I am not even sure if it is a software or hardware problem.
The machine does not crash and remains under software control at all times (eg num lock always works, ctl+alt+del will cause an orderly reboot) but the boot does not complete: I get to "/scripts/init-bottom ... done", few minute wait, then get messages like "scsi: killing requests for dead queue", then a message to say one of my SCSI tape drives has been attached then, again after a short wait, some messages about how "watershed sh -c '/sbin/lvm/vgscan; /sbin/lvm vgchan vgchange -a -y'" has been terminated with signal 9, then a "done" and then nothing - ie the machine just sits there, still under software control but apparently doing nothing.
Hitting ctrl+alt+del gives a message that alsa-main process and plymouth-upstart-bridge are being halted before an orderly reboot begins.
If I use a rescue CD the filesystem seems to be there undamaged.
I know this is all a bit vague, but any help at all with this would be very welcome.