I have my syncocs (I believe) to update only, and do a full sync my preferences are below:
Customised sync control
Upload local files to google: Yes
Download google files to this PC: No
Sync renames, moves and deletions too (do a full sync): Yes
backup google contact list once a day: No
Sometimes syncdocs deletes my local C:\ drives files even though I have it set to upload only, as yet I cannot find any link between what files it decideds to delete and which ones do not.
I have looked at modified dates / times as well as file type etc and so far it seems completely random.