Well, having deleted all the music from the K drive and resynced, the display is looking more sensible (it's no longer claiming there is stuff to sync and the size / free space are more accurate). I had forgotten about some recordings that I hadn't accounted for, but there wasn't 2.4GB worth (just about 120MB, I didn't delete any music from elsewhere). There is now about 49.5 MB of other files on the K drive (according to WinDirStat), but MM reckons there's 300MB. There's 972MB free, but MM calculates 987MB (I guess that could be a difference between used on disk and actual data? or not accounting for the filing system itself?).
Either way, the L drive still seems a bit odd. I haven't tried clearing it because I'm intrigued at what's going on. It's as if the algorithm to calculate the bar at the bottom is thinking there is audio still to be synced, but once the sync algorithm runs it realises those files actually exist. As I've said above, even after I sync it still thinks there's files to sync (currently 1.7GB, up from 550MB when I posted the last screenshot). As you can see below, I only have two folders (music and video), no files in the root and 'delete files not included...' is ticked. Some of the music on it may well have come from an old version of MM but this set-up should sort it out. After a sync (which calculated for a while then stopped without transferring anything or giving any messages), I've got 5353 music files on the device, but the bar in MM calculates 5570. In the auto-sync list if I hover over the node it also reports there are 5353 files to be synced.
As above, the 'other' amount seems quite a way out. I even tried changing the music directory from all caps in case it was a continuation of that bug, but no luck.
One other thing, I noticed that the white space behind the text for the three text boxes, 'scan as soon as...', 'auto-sync...' and 'automatically unmount...' will go over the device image when the window is made small, even though the text is nowhere near it. Don't know if this is to accommodate much larger text sizes.
I thought I might have figured out at least part of what was going on, the tracks have been running volume analysis and I thought it might be to do with that. But the main 'album' for which that's been running is 338 files, but the difference between reality and what the bar thinks should be happening is only 217 files.