Thanks - to answer your comments / questions
"Windows does not hibernate external drives."
True, however, Seagate provide a utility so you can tell the HDD to go to sleep after inactivity - I've set it to 15 minutes so having left it to copy 4TB overnight it was sleeping when I came back, turned the PC off and disconnected.
"Unplugging the drive during write operations."
Windows always tell it to eject or close PC down. RPi4 power off machine, but it shouldn't really be writing to the disk anyway - just reading videos.
"Insufficient power for write operations."
I'm pretty sure this was the first cause - power block with duff socket.That was fixed but I suppose its possible that the Pi was overloaded, couldn't read the OS from SSD and tried to create a new install on the HDD.
"Vibration"
Being a brit a tumbler is a glass (one of those things you drink out of) so might have trouble balancing the HDD on it
"Disk failure"
I'm about 23% through a full scann (HDDScan) right now - no bad sectors yet.
"Does it corrupt after being remove from Windows, from libreelec,
or during operation on one of the systems?"
Good question - I only found it (both times) when I plugged back into my Windows PC - massive index corruption both times.
"How does corruption show up in the kernel log? (dmesg)"
No idea - the SSD where I presume it would be stored has been flashed several times since it happened.
"PS: Why does the forum censor the word "
Wish I knew - it also censors f00d
My PS: any views on ExFAT vs NTFS?