Playlists empty

Hi

I'm running LMS 8.1.1 on Windows 10 on a new PC. The MP3 files are on a portable USB HD ( M: ) transferred from my old PC.
Today I notice all my playlists are 'empty', even those newly created directly with the LMS web interface if and when the playlists are re-scanned.
It seems the act of re-scanning a playlist causes LMS to believe it to be empty. For example, this file created by LMS after re-scanning will not play. Rescanning doesn't alter the M3U file.

#CURTRACK 0
#EXTM3U
#EXTURL:file:///M:/MySong.mp3
#EXTINF:200,My Song
M:\MySong.mp3
...etc.

I tried re-writing the M3U file to the old style, e.g.

#EXTM3U
#CURTRACK 0
#EXTINF:-1,My Song
M:\MySong.mp3
...etc.

but still treated as empty by LMS.

Maybe this is a file access permissions matter? Although I do have all media files' permissions set as full read/write for the user 'Everyone'.

Any ideas? Thanks

GNQ