Found a couple of cautionary notes on an Apple support page:On a Mac with Apple silicon, starting up from an external storage device requires a working built-in startup disk. If your built-in startup disk is experiencing a hardware failure, you can’t use an external startup disk instead.
If your built-in startup disk has a different major version of macOS than your external startup disk, be aware that a more recent macOS version on one disk can install changes designed to keep your Mac secure, and these changes can affect your computer's ability to use a significantly earlier version of macOS on the other disk.
The first isn't really an issue (not what I'm using it for), but the second is potentially problematic- depends what they mean by "significantly earlier".... not keen that the Neo could get bricked (even as a remote possibility) just because I'm trying out a newer OS on an external drive 
(Yes, I realise that booting into recovery mode would probably still work, but on a risk/benefit balance it's risking a lot of faff just to satisfy a mild curiosity
)
I presume the security thing is the firmware update that a newer OS would update the Mac to, this is one of the things I was a bit worried about when changing to a Mac. Never had it on windows, I updated the bios, when i wanted to and it certainly did not bother about going back from 11 to 10 or even 8 if I wanted to.
I realise the Neo uses a different CPU than other Macs, not sure how much different it is to the M chips.
I changed my Mac to boot from he external hub drive, turned the Mac off and disconnected the hub. Turned the Mac back on and eventually recovery mode popped up and gave me the choice to boot from the internal drive./
Turned it back off, connected up the hub and now using GG on the external drive.
I doubt installing GG to a external drive will brick your computer, but it is your choice, it is fine on my M2 pro. I installed GG on the external drive and then at some point went from Tahoe to Sequoia on the internal drive. Still fine. Now back to Tahoe on the internal.
I also did a bit of checking to what version of the OS the recovery mode would want to reinstall and it turns out it depends on what drive I boot up with. If i boot up with the external with GG, then it would reinstall GG, if the internal with Tahoe, it would reinstall Tahoe. So it seems the recovery mode goes by what is installed. Not sure what would happen if I wiped the drives, but not going to do that, certainly not the internal one.
I am using GG on the external drive now, , it is okay, just a bit slow, reminds me of my PC
As I said it is your choice. the firmware version of my Mac is 20457.41.3
Adrian
Desktop machines Mac mini pro with macOS Tahoe, also pc Ryzen powered with windows something or other.
Zooming with Zzoomm FTTP,