A workaround of course is to leave some capacity unused. I haven't yet found out how much this needs to be.
Finding out how much this needs to be seems rather difficult. The answer depends on space used by additional objects created during the sync, inc. playlist files, folders, .ALB files, and (to a lesser extent) JRiver's own files JRInfo.xml and JRState.xml.
FWIW, below are two success cases. Based on those, my proposed rule of thumb to avoid incomplete sync: For a sync list having approx. X tracks and approx X/10 playlists having approx X track references in total, ensure that JR MC device window shows an amount available which is at least 7% of the device's formatted free space figure.
Creative ZEN 8Gb model, firmware V1.21.03e name "ZEN G", no user wallpaper, newly formatted.
Before sync:
Device shows Free space: 7.62GB
http://img20.imageshack.us/img20/4213/1h6.pngMC 18.0.206 with a sync list containing 1953 (track) files (7.1Gb) plus 182 playlists containing a total of 2182 track references. MC device window says 566 MB available
http://img12.imageshack.us/img12/3639/cn3v.pngAfter sync:
Sync success confirmed by device UI saying Track Count: 1953, Playlist Count: 182. Also: Free Space: 45MB, Album Count: 107, Photo Count: 0, Video Count: 0
Device's Explorer Properties says Used space: 7.44GB and Free space: 188.0 MB
http://img849.imageshack.us/img849/6798/5kic.pngCreative ZEN 8Gb model, firmware 1.21.01e, name "ZEN A", user wallpaper, newly formatted.
Before sync:
Device shows Free space: 7.49GB
http://img707.imageshack.us/img707/156/gc0j.pngMC 18.0.206 with a sync list containing 1953 (track) files (7.1Gb)
http://img826.imageshack.us/img826/9711/yzyi.png plus 182 playlists containing a total of 2182 track references. MC device window says 423 MB available
After sync:
Sync success confirmed by device UI saying Track Count: 1953, Playlist Count: 182. Also: Free Space: 188Mb, Album Count: 107, Photo Count: 0, Video Count: 0
Device's Explorer Properties says Used space: 7.44GB and Free space: 45.0 MB
http://img196.imageshack.us/img196/3840/3b91.pngNote: MC device window "X MB available" means X Mb remaining free after the sync.