Unable to write to Storage via Samba or SFTP

I have discovered a problem whereby in trying to transfer files (e.g. to update, backup or even userdata) via a Samba or SFTP mapped drive - it says there is not enough space - in fact it says there is ZERO space for a file transfer.
Then I tried using SCP command from a Powershell Terminal - this worked with no problem
I can also ‘write’ from within CoreElec itself, whether that be sourcing an ‘update’ or creating a ‘backup’ - these are added to the appropriate folders without issue and once loaded I can see them using the STFP map in Explorer
So what has gone wrong with the access to write to that storage drive? I also tried to access it when booting into the main Android system - again I can see what is in there, copy FROM but not TO!
I wasn’t sure if this was initially my SD card - or the USB Card Reader - but tried a couple of different cards and also just a plain USB thumb drive - all behave the same in regard to writing to the Storage Drive. Note that my error is NOT that I do not have access privilege in any way, it says there is zero space available.
I know this worked fine on previous installs - what is going on that may have changed?

EDIT - OK - sorry, I found COULD access when simply mapping the drive via the QuickAccess in FileExplorer - but does not access via the SFTP or directly when drive in another device
Can someone explain that mechanism to me?

Obviously some kind of SFTP program issue (this was the SFTP Drive V2 program that I saw recommended elsewhere on this forum)
I just tried this - http://www.coreftp.com
It has a dual-pane display for your server and client
It looks good!

I use WinSCP

Yes, tried that too earlier today - searched everywhere on how to show the hidden folders - checked it again just now and saw it immediately down in the bottom right corner :flushed: