I have a grub-legacy setup on a usb drive.  I've dual booted windows and multiple linux distros long
enough to not trust anyone with the boot partition, so I leave it unmount/unplug when installing or
upgrading.  It's saved me a lot of hassle with things like this.  As long as you don't have anything
weird or fancy going on, and your bios is sane, it works.

Haven't tried it with a grub-2 setup, but it should work.  Once you have a stable setup and
everything works, it's just a matter of copying over the latest kernels.

On 09/14/18 06:54, o1bigtenor wrote:
> Greetings
> 
> I have had it working before but I can't seem to get things right this time.
> 
> I want to have two different copies of debian on one box with the
> choice of which when I boot in.
> Wanting to have available both debian stable (9.5 at present) and
> debian testing (10). This is so I can experiment with software - - -
> adding it and if huge errors result or are caused - - - no real biggie
> because its not a main working box (like the server or my main
> computer). Doing this because I really have gotten to hate having a
> main box down for even a few days because software that I loaded and
> installed caused me to bork the system (I've done this more than a few
> times!!! grin - - -but 'learning' isn't always a barrel of fun!!)
> 
> So I've installed both of these systems (more than once each) they
> have their own partitions for everything but boot and efi yet I'm only
> seeing one system available on grub (depending upon the last install
> as to which). So I'm doing something wrong!! I tried using grub
> updating tools (# os-prober) still no joy. The web pages that I'm
> finding seem to be for an older version of grub and, as usual, I'm
> finding man pages are like reading cuneiform (which I find
> unintelligible).
> 
> This is likely something quite simple but I'm just not seeing it - - -
> please - - - some ideas/pointers?
> 
> TIA
> 
> Dee
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