Sorry about that...

GA = Guest Additions

Both distros have GA installed, so to speak, because I can run the distro at full resolution (that matches my Monitor's resolution. (rather than 640 x xxx)

Is the how to guide referencing a windows to guest shared folder? Versus sharing the folder within the two distros running in the virtual box...

I guess I was referencing a Windows PC. If you select automount it will automatically show up under /media.

Guys have reported (in the thread) that they were able to move a file/folder between distros. I don't know how they did it.

No, full screen of VM client does not mean that GuestAdditions software is installed. Current linux kernels provide video support for VirtualBox clients:
This has nothing to do with installing GA.
Unless you install GA ( read how to do it) clipboard copy/paste function will not be enabled, also sharing folders/partition/disks will not work either. CD attached to virtual drive is no the same as installed software.
Sharing folders with Windows VM client and linux/BSD VM clients requires different setup.
what is the output of:
in you linux VM client?

There are actually many ways other then shared folders. It depends on how their network adapters are configured.

ftp
sftp
scp
nfs
samba (Windows networking)
If the virtual machine is configured with a CD drive an ISO file.
netcat
You can probably transfer files via a virtual serial/network port.
email if configured and the files are small
cloud service of some type if configured.
web server if configured

etc...

ubuntu@ubuntu-VM:~$ lsmod | grep vb
vboxvideo 36864 0
ttm 102400 2 vmwgfx,vboxvideo
drm_kms_helper 180224 2 vmwgfx,vboxvideo
drm 483328 8 vmwgfx,drm_kms_helper,vboxvideo,ttm
vboxguest 335872 5
ubuntu@ubuntu-VM:~$

P.S. The command posted above was copied from Firefox (running in Windows) to Ubuntu in the VB... Then the Output was pasted back to the forum, (from the Guest to Windows...)If that helps.

O.k.
so you have installed standard but crippled version of VirtualBox Additions for client that a lot of distros provide automatically - missing vboxsf module for (for mounting shared folders)

You will not be able to use Virtualbox file sharing utility but you can use third party methods as suggested by michaelk.

I don't know if your clipboard copy/paste facility should work or not, I always install default full version of VirtualBox Guest Additions. It is more convenient.

To install full version of VirtualBox additions you must have kernel headers installed first (provided by your distro that is running in VM as guest).

If you want to try it, let me know, otherwise you can use ways for sharing files as suggested above.

How do you do install vboxsf?

Reinstall GUest additions I would say.

If a dependency was missing I would expected the GA installer to fail and post an error message.

See this link.

https://www.tecmint.com/install-virt...in-ubuntu/amp/

I would not call the methods I posted as third party...

If you don't have the kernel headers installed when you run the GA installer, it will install but not be able to build the driver for vboxsf. You'll get an error message at the end saying "modprobe vboxsf failed". All the usual GA goodies will work except for file sharing via the shared folder. That's probably what you've run into. Following the directions in michaelk's link should correct that problem.

But that's not the end of things. The above will insure you create the vboxsf module but only root will be able to access the shared folder in your linux guest. To get around this permission problem you have to add your user to the vobxsf group with:
You also have to change the permissions on your linux guest of the shared folder mount point which will be at /media/sf_<shared folder name>. For example, if you created a shared folder in your home directory named "public" you would find it mounted on your linux guest at /media/sf_public. If you navigate there, you will see this directory but be locked out. Using this example, you would change that with:
You then need to reboot your linux guest for these changes to take effect.

Well it depends on your skills so think about this as if you don't know much about linux yet, after spending confusing time trying to make it (vboxsf) work, you may be left without file sharing anyway.

I just want to stress out that for the beginner it may not be that easy although soon you will realise that in fact this is quite simple.

Anyway I will give you some first hints and you decide what do you want.

first few questions:
1) what is your VM host? linux or windows?
2) what VM clients are you running?
3) what version of Virtualbox is your host running?

next:
1) install headers for the kernel you are currently running by your linux VM clients.
to check kernel version, in your linux client VM open terminal window and run:
you have to install exactly the same version of headers as your kernel. This is important because most of the distros provide for installation more than one kernel so headers and kernel versions must be the same
2) download VBoxGuestAdditions_xxx.iso from here
https://download.virtualbox.org/virtualbox/
select version corresponding to your host version.
That would be the easiest although host VM and client VM does not have to match.
If you have problems just download:
https://download.virtualbox.org/virt...ons_6.0.14.iso

let me know how do you plan to proceed or if you want to try, if succesfully installed headers and downloaded (by your host) VBoxGuestAdditions_xxx.iso

VM Host: Windows 10 1903

VM Clients: MX Linux (where my .odt is) and Ubuntu, (only because, an acquaintance of mine said he runs it because there's a much higher level of support, than with MX Linux. And to his point, another friend of mine from another forum, asked, "Where do you guys hear of these distributions?" i.e. he never heard of it before. So, I can understand the first gentleman's point, regarding the ubiquitous-ness of Ubuntu. (hence he'd receive "better" support, because more users are using Ubuntu. (That's his thinking anyway.)

Version of VirtualBox: 6.0

o.k. so install headers as mentioned above and download
https://download.virtualbox.org/virt...ons_6.0.14.iso
on your windows host

let us know when you are done

michaelk, sorry for the delay...

GA = Guest Additions

In the meantime, (since I last posted,) I hve tried, and tried to get GA to work, but to no avail.

Changing gears, to my latest idea...

What if I had side by side installation with my Windows OS? I have heard it said, that if you ever wanted to return your system to a Single Boot Environment, it would be very, very difficult to do. Hence, my resistance to going the dual boot method.

However, I do have backups of my Win10 1903 OS as recently as this morning (11.29.2019). So, I was wondering: if the GRUB boot loader went kaput, couldn't I just restore a backup using all of my "C" drive and re-install Linux from beginning? So, at this point, the drive would be returned to its state on 11.29.2019, and the only thing left to do, would be is: re-install Linux. Would that work in a worst case scenario?