That's interesting, about the TPOS frame. Didn't consider that. The audio source, however, is FLAC, where the DISCNUMBER does not have such limitations.
(Does MM enforce tag rules, such as those suggested by BKKKPewsey?)

I exported the device profile. You can find it here: http://www.2shared.com/file/Mx4vF7S3/MyDeviceProfile.html. Maybe that'll help recreate the issue?
Additionally, to make things worst - I don't keep tag synchronized. And in that scenario I described, the <Disk#> tag was edited (=out-of-sync).

[quote="MeeMeeMee"]while for others it doesn't (<Disk#>="bonus disk")[/quote]

That may be the problem as I believe that tag should have only a numeric string eg 1 or 2 which can be extended with a"/"
Very similiar to the track no. tag TRCK
From the ID3 web site
[quote]TPOS
The 'Part of a set' frame is a numeric string that describes which part of a set the audio came from. This frame is used if the source described in the "TALB" frame is divided into several mediums, e.g. a double CD. The value may be extended with a "/" character and a numeric string containing the total number of parts in the set. E.g. "1/2".[/quote]
:)

Step 4b: http://www.mediamonkey.com/forum/viewtopic.php?f=7&t=69

When I synchronize music to my Cowon D2 16G (using d_WMDM.dll plugin), I use the following Destination mask:
[code]\Music\<Album Artist>\[<Year>] <Album>\$if(<Disc#>,<Disc#>)\<Track#:2> - <Title>[/code]
FLACs are converted to MP3 (Extreme: ~256kbps...)
Also, I sync artwork to folder: cover.jpg, don't copy artwork to tag and strip artwork from tags.

Everything works as expected with copying artwork in the following cases:
1. When the <Disk#> tag is empty.
2. When all the tracks in the album has <Disk#> set, as in two-album disks, where some have 1 and some have 2. Then, the artwork is copied to both folders.

It doesn't work well when for some tracks <Disk#> is empty (=the album tracks), while for others it doesn't (<Disk#>="bonus disk"). I have a few special edition albums with an additional bonus disk where I use this sort of setup.
For these albums, the artwork isn't copied into the [i]\Music\<Album Artist>\[<Year>] <Album>\<Disc#>\[/i] folder.

I'm using b1470. Maybe this should be moved to the beta forum.