Ugh. A terrible bug* indeed, but it also seems like Sugarsync doesn't support one-way sync... which does make sense, and it's important to note that there are different types of cloud hosting providers - some are "store stuff in the cloud and access from anywhere" (think iCloud, Dropbox, synchronizing in both directions), some are more for backup - nothing more than a remote hard drive with no built-in magic. For important backups, I'd be wary of the former kind.
* Although they do
write: "It's very important that you sync data stored on internal hard drives. Syncing from external or removeable drives is not supported by SugarSync." (still, no excuse)
I have a couple of external drives stored in two different locations that I update every now and then. I'm going away for a while so I'm just about to set up some online backup for the Really important stuff (e.g., the _good_ pictures, which shouldn't take up much space...). I'll dump it to Amazon S3, or maybe use
TarSnap (encrypted backup built on top of S3).