Although i did manage to install MX KDE to a usb connected SSD and have it run on any computer (without installing GRUB to MBR), i would like to do the same with KDE Neon which is rolling distro. Can it be done?

Here is procedure used for MX KDE:

Boot in UEFI from a MX LiveUSB.

Prepare partition layout of the external drive in Gparted:

1st: Device -> Create Partition Table: GPT ( not MSDOS !)

2nd: Create partition: bios_grub (this is needed to smoothly allow BIOS booting on GPT):
Size: 1MB
File system: unformatted (scroll down to last entry),
Partition name: bios_grub
Label: leave empty
Click: Apply all Operations!
Right-Click or Menu->Partition: Manage flags: select "bios_grub" (important!)

3rd Create partition: ESP (EFI System partion"
create: 512 MB, format Fat32
Partion name "espUSB", File system label "espUSB"
Manage flags: select "esp+boot" (important!)

4th Create further partitions as needed :
next partitions:
boot ext4 512MB (only needed for encrypted install,otherwise ignore)
root ext4 32GB or bigger
home ext4 nnGB
label all partitions accordingly
E.g. I used rootUSB, homeUSB

Now run the "MX Installer"

and select the prepared partitions for root, and home on the USB device
On GRUB install select "ESP" and partition "espUSB"
When the Installer has finshed do not reboot yet "un-select" to reboot on the last screen.

Now, to prepare BIOS booting:
Open "MX Boot Repair"
and install Grub loader for BIOS booting
Only select "re-install" GRUB, and select MBR of the USB-device.

- - - - -

Did post to KDE Neon forum but no one is picking up on the thread.

Basically, yes, you can install any Linux distro onto an external, USB-connected hard drive, rotating or solid.

Great!!

Think the last paragraph of my posting was where i couldn't go any further.
"MX boot repair" doesn't evidently exist in other distros.
Hence the next question: how to go about installing Grub loader for BIOS booting etc.?

If you want to be able to boot on other machines, you must install grub into the MBR (or some equivalent) of the SSD, as the message says.
Where's the question/problem?

Actually, i had never done kind of thing before.

It just worked out fine and thought that the last, MX installer part, was kind of specific to MX abilities.

Could an update cause issues with booting on a PC?