I have been using this function to create and maintain a matching mp3 library for my lossless files.
Today when I went to Sync (so new rips will be put onto the 'Handheld' drive) MC froze as it started the conversion process and I had to force it closed before it had sync'd any files.
After re-starting MC I again went to attempt this and this time MC reported that the "Handheld is fully sync'd", in spite of the fact that there had been numerous files to sync prior to the crash (and none had completed before the freeze-up).
In an attempt to 'fix' this I ran an Import, which oddly enough showed MC "Updating to match external changes" after which the import completed and showed MC had removed numerous "Broken Links".
I had run an import just prior to my first sync attempt (the attempt where MC crashed), and I had NOT manually deleted, changed, or touched any files since (and no other software was running that could have). The only thing that had transpired in between was MC's accessing the drive to attempt the sync.
Now MC shows I have 8.5 hours of convert/sync to complete the process when before the crash it had only showed 2.3 hours.
Apparrently MC actually deleted files in that first sync attempt when it crashed, and based on the difference in times I expect that many of those were not meant to be deleted.
Also, I have seen MC erroneously show the "Device Fully Sync'd" message when I attempt a sync although I know this not to be true. Usually a re-start of MC or a Library Import will correct this and show the correct nimber of files to be sync'd.
Perhaps this is related to what transpired today.
In any case, I suggest using caution if your "handheld's" files are not backed-up as it appears that some may be lost.