Grub, Grub2, or your UEFI BIOS (if you have one) can all provide a menu
for what to boot.
I hear your pain. I am currently running Debian-based Ubuntu 16.04.5
LTS on a Dell XPS 15 9650 4K LCD with a Dell TB16 dock - all is fully
functional except dimming the external 34" or 40" 4K dock attached
monitor. It took me a bit to get this all working and Ubuntu 18.04.1
LTS replaced Unity with Gnome 3 which also removed franctional scaling
of the display(s) - only providing for 100% or 200%, and I currently
run at 150% or 162%. Gnome 3 also screws up my virtual workspace
preferences and talk on the forums is that this hardware combination is
borked on 18.04.1, so I'm right there with you.
I will make a dd replica of my 16.04.5 SDD onto one or more external
SDDs and play with upgrading them in place. Grub2 should automatically
add the new external SDD to the boot menu.
On my Ubuntu desktop workstation, I have space and some open SATA ports
that I can simply plug a fresh SDD into and do a fresh install. I would
let Grub2 update the MBR of the existing bootable partition.
I have had little to no issues with Grub or Grub2, so I continue to use
that for my boot menu and boot options.
It sounds like you have split a single HDD/SDD into multiple partitions
for your different OS versions. Do you have the ability to have
separate disks for each OS version? That shouldn't be needed, but one
never knows. I always use separate disks when I do these things.
On Fri, 2018-09-14 at 06:54 -0500, 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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