Failure of reboot after update

[Xen]

bmarsh at bmarsh.com schreef op 12-10-2016 0:49:

>> It is almost impossible that Kubuntu could cause your system to hang 
>> at the Toshiba screen because normally whatever goes wrong you should 
>> *always* get a grub prompt.
> 
> Not necessarily so.   When I put the latest ISO to a USB stick, I have
> three choices of the utility used to build that stick:  unetbootin, or
> the "make starter disk" with either gtk or kde?
> 
> It usually takes me a couple tries to find the right one...  and on
> the failures, I do not get a grub boot but rather a blank screen.

The ISO does not use Grub to boot itself. Further I was talking about 
the base system, installed system. It stands to reason that you could 
prevent your system from booting by inserting some malfunctioning USB 
stick. But that is only temporary. Your system should boot or at least 
give a semblance of a Grub prompt simply because there is virtually 
nothing that would *wipe* the bootsector or the location of the grub 
image (boot.img and core.img). Since Grub is embedded in the bootsector 
(boot.img) and normally an installation cannot prevent the BIOS from 
reading the bootsector of the harddrive there is simply nothing your 
system can do to prevent your BIOS from reaching that bootsector other 
than wiping it clean. If there is one thing few tools do by default then 
it is clearing the bootsector. Grub certainly would not (it would not 
wipe its part from there and then fail to install a new one).

So generally speaking you cannot have a failing system that will not 
boot but will also not /at least/ load Grub.