G'Day Michael,
I didn't notice for a while that I got a reply (thought I would get an email or something).
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I am using 23.0.102 (64 bit) version.
That should work fine as your Sony Walkman NW-ZX100 is visible as a drive letter in Windows.
I do notice in the "to do" list for the sync, it follows a pattern of wanting to "QUEUE" a file with path "z:\music\etc\etc\.flac" (the path to my music share) and "delete on sync" the identical track but with file name "f:\music\etc\etc\.flac" (the path to my handheld).
If you see that, check the Date Modified for some files on your PC versus the files on the device. Different? Maybe the device is changing the Date Modified when the files are saved to it. I don't think MC does. But check if there is a difference.
Note that if you change a tag that is stored in the file, then the Date Modified will be updated, and hence the file may be synchronised again, depending on that setting. You can easily tell which tags are stored in files by going to Options > Library & Folders > Manage Library Fields > then click the dropdown list in the top left corner and select "Show only fields saved in tags".
I don't sync a large number of files, so there could be an issue when that is the case. I haven't seen anyone say they have that issue, but there are quite a few threads on Handheld Sync about issues with the functionality. Maybe that has been an issue for others, but they haven't noticed.
It would be great to get some insight into how MC determines if something needs re-syncing, e.g. is it file name, track name, etc.
I'm pretty sure it is just;
Is the track (Library record of the track, which includes all the definition of the track) in the sync list, via a Playlist or manually added via Send To, Drag and Drop, or other methods,
does the file exist on the device, using the [Filename (name)] tag and the File & Path rules for the device,
and is the Date Modified the same on the PC and device,
hence, decide whether to sync (add, delete, or both to update) or not.
That is about all I have. The functionality seems pretty robust to me, although sometimes a little confusing, as shown by the thread I linked to above. Worth a read through if you haven't. If it really is just deciding to resync a large portion of the files again for no apparent reason, that is a new issue to me. But the Handheld Sync functionality is still incomplete and being worked on in the 64bit version of MC, so if the issue can be replicated by JRiver they will fix it, eventually.