RPi4, 12/9/22 nightly, "FILE TO LARGE"

I'm having my latest attempt to switch to the RPi4. It seems to have gone all right (to early to really tell yet) apart from one weird phenomena. In music videos I have a (downloaded from YT) small clip from Medieval Babes. The title is Gaudete. When I tried to play it I couldn't and investigating I found the mp4 was missing (the jpg & nfo are there). When and why the mp4 isn't there I'm not sure - it is on my backup disk which was refreshed this morning whilst I was still on the RPi3 so I suspect importing the music videos did it (whatever it is).

I thought - OK I'll copy it back - which is where I get the weird message that the file is to large - we're talking about a roughly 5MB file on a 6TB disk with a couple of TB spare!

Even weirder if I change the name from Gaudete.mp4 to xxGaudete.mp4 it copies across just fine.

I don't know if any other files are missing yet - I'm off to write a small program to check everything out.

Anybody any ideas?

I don't wish to be contentious but Windows & RPi3 LE 9.2.8 (I think) have worked fine since 2018. Switching to RPi4 & LE10 - not handling mp4 file correctly, moved to LE11 CEC problems & SAMBA problems. Or to put it another way I don't think its Windows fault.

I have FileZilla (similar to WinSCP I assume) but, unfortunately, I didn't think to try it. But a simple disk copy should be no problem especially with oodles of space left.

The 6TB disk is attached to the RPi4 by USB cable into one of the USB3 ports, the SSD holding LibreElec and Kodi is in the other. The filesystem is bog standard NTFS - again working perfectly since 2018 on the RPi3. The disk is the one that was corrupted several times but just worked happily when connected to the RPi3 for the last few weeks.

I have no idea how to investigate. If you have any suggestions I can follow I'm interested.