In the meantime, I'm going to backup my current install of Kodi and then see if disabling NTFS3 (https://forums.linuxmint.com/viewtopic.php?t=398433) and using NTFS-3G will resolve this problem.
...well that's a wash.
Doesn't look like I can 'disable' NTFS3 in favour of NTFS-3G. Apt-get commands are ignored and it appears that LibreElec prevents you from making changes to the system.
I couldn't figure out how to permanently set NTFS3 to 'Read-Only' for all NTFS mounted drives as an alternative. Someone cleverer than I would have to offer their assistance in this regard, *if* the write aspect is even the culprit. I would have liked to test this theory as it seems that the NTFS-3G default of setting NTFS drives to Read-Only was somewhat more forgiving.
Crystal mark, chkdsk indicates both 12v HDDs are error free when scanned.
I don't see that there is anything 'wrong' necessarily in the way I have the system set up; it's maybe just a little inefficient... or maybe not, I don't know (?) It is however, something I understand.
Anyway, I have ordered a new HDD, will format that to ExFat and carry on.