Hi Jim,
I'd be very surprised if the drive letter was changed by Windows. I have a pretty good understanding of what can trigger that. For example, if you plug a USB drive into the computer, and that drive hasn't been assigned a letter yet, then Windows might assign it the letter of a mapped network drive, if the network is offline. But that didn't happen. Besides, if that somehow were the case, then I would expect that all the files would have been re-imported, not just 2/3 of them, since all my files are in the same sub directory on the same drive. Unless Windows changed the drive letter and then unchanged it while MC was doing the import... And if that happened, then the newly imported files would not play, because the drive letter in their path would be wrong. But they did play.
Thankfully for the automatic backups, I didn't loose more than 2 days of play statistics. But I was lucky to have noticed before losing more than that.
Is there any other scenario where MC might re-import files? Not, by chance, if an import rule which writes a tag value, changes?